Mr. Sepi did not like to venture outside too late. But, plagued by nightmares about an Iraqi civilian killed by his unit, he often needed alcohol to fall asleep. And so it was that night, when, seized by a gut feeling of lurking danger, he slid a trench coat over his slight frame — and tucked an assault rifle inside it.

“Matthew knew he shouldn’t be taking his AK-47 to the 7-Eleven,” Detective Laura Andersen said, “but he was scared to death in that neighborhood, he was military trained and, in his mind, he needed the weapon to protect himself.”

Head bowed, Mr. Sepi scurried down an alley, ignoring shouts about trespassing on gang turf. A battle-weary grenadier who was still legally under-age, he paid a stranger to buy him two tall cans of beer, his self-prescribed treatment for post-traumatic stress disorder.

As Mr. Sepi started home, two gang members, both large and both armed, stepped out of the darkness. Mr. Sepi said in an interview that he spied the butt of a gun, heard a boom, saw a flash and “just snapped.”

In the end, one gang member lay dead, bleeding onto the pavement. The other was wounded. And Mr. Sepi fled, “breaking contact” with the enemy, as he later described it. With his rifle raised, he crept home, loaded 180 rounds of ammunition into his car and drove until police lights flashed behind him.

 “Who did I take fire from?” he asked urgently. Wearing his Army camouflage pants, the diminutive young man said he had been ambushed and then instinctively “engaged the targets.” He shook. He also cried.

“I felt very bad for him,” Detective Andersen said.

Nonetheless, Mr. Sepi was booked, and a local newspaper soon reported: “Iraq veteran arrested in killing.”

Town by town across the country, headlines have been telling similar stories. Lakewood, Wash.: “Family Blames Iraq After Son Kills Wife.” Pierre, S.D.: “Soldier Charged With Murder Testifies About Postwar Stress.” Colorado Springs: “Iraq War Vets Suspected in Two Slayings, Crime Ring.”

Individually, these are stories of local crimes, gut-wrenching postscripts to the war for the military men, their victims and their communities. Taken together, they paint the patchwork picture of a quiet phenomenon, tracing a cross-country trail of death and heartbreak.

The New York Times found 121 cases in which veterans of Iraq and Afghanistan committed a killing in this country, or were charged with one, after their return from war. In many of those cases, combat trauma and the stress of deployment — along with alcohol abuse, family discord and other attendant problems — appear to have set the stage for a tragedy that was part destruction, part self-destruction.

Three-quarters of these veterans were still in the military at the time of the killing. More than half the killings involved guns, and the rest were stabbings, beatings, strangulations and bathtub drownings. Twenty-five offenders faced murder, manslaughter or homicide charges for fatal car crashes resulting from drunken, reckless or suicidal driving.

About a third of the victims were spouses, girlfriends, children or other relatives, among them 2-year-old Krisiauna Calaira Lewis, whose 20-year-old father slammed her against a wall when he was recuperating in Texas from a bombing near Falluja that blew off his foot and shook up his brain.

A quarter of the victims were fellow service members, including Specialist Richard Davis of the Army, who was stabbed repeatedly and then set ablaze, his body hidden in the woods by fellow soldiers a day after they all returned from Iraq.

And the rest were acquaintances or strangers, among them Noah P. Gamez, 21, who was breaking into a car at a Tucson motel when an Iraq combat veteran, also 21, caught him, shot him dead and then killed himself outside San Diego with one of several guns found in his car.

Tracking the Killings

The Pentagon does not keep track of such killings, most of which are prosecuted not by the military justice system but by civilian courts in state after state. Neither does the Justice Department.

To compile and analyze its list, The Times conducted a search of local news reports, examined police, court and military records and interviewed the defendants, their lawyers and families, the victims’ families and military and law enforcement officials.

This reporting most likely uncovered only the minimum number of such cases, given that not all killings, especially in big cities and on military bases, are reported publicly or in detail. Also, it was often not possible to determine the deployment history of other service members arrested on homicide charges.

The Times used the same methods to research homicides involving all active-duty military personnel and new veterans for the six years before and after the present wartime period began with the invasion of Afghanistan in 2001.

This showed an 89 percent increase during the present wartime period, to 349 cases from 184, about three-quarters of which involved Iraq and Afghanistan war veterans. The increase occurred even though there have been fewer troops stationed in the United States in the last six years and the American homicide rate has been, on average, lower.

The Pentagon was given The Times’s roster of homicides. It declined to comment because, a spokesman, Lt. Col. Les Melnyk, said, the Department of Defense could not duplicate the newspaper’s research. Further, Colonel Melnyk questioned the validity of comparing prewar and wartime numbers based on news media reports, saying that the current increase might be explained by “an increase in awareness of military service by reporters since 9/11.” He also questioned the value of “lumping together different crimes such as involuntary manslaughter with first-degree homicide.”

Given that many veterans rebound successfully from their war experiences and some flourish as a result of them, veterans groups have long deplored the attention paid to the minority of soldiers who fail to readjust to civilian life.

After World War I, the American Legion passed a resolution asking the press “to subordinate whatever slight news value there may be in playing up the ex-service member angle in stories of crime or offense against the peace.” An article in the Veterans of Foreign Wars magazine in 2006 referred with disdain to the pervasive “wacko-vet myth,” which, veterans say, makes it difficult for them to find jobs.

Clearly, committing homicide is an extreme manifestation of dysfunction for returning veterans, many of whom struggle in quieter ways, with crumbling marriages, mounting debt, deepening alcohol dependence or more-minor tangles with the law.

But these killings provide a kind of echo sounding for the profound depths to which some veterans have fallen, whether at the bottom of a downward spiral or in a sudden burst of violence.

Thirteen of these veterans took their own lives after the killings, and two more were fatally shot by the police. Several more attempted suicide or expressed a death wish, like Joshua Pol, a former soldier convicted of vehicular homicide, who told a judge in Montana in 2006, “To be honest with you, I really wish I had died in Iraq.”

In some of the cases involving veterans of Iraq and Afghanistan, the fact that the suspect went to war bears no apparent relationship to the crime committed or to the prosecution and punishment. But in many of the cases, the deployment of the service member invariably becomes a factor of some sort as the legal system, families and communities grapple to make sense of the crimes.

This is especially stark where a previously upstanding young man — there is one woman among the 121 — appears to have committed a random act of violence. And The Times’s analysis showed that the overwhelming majority of these young men, unlike most civilian homicide offenders, had no criminal history.

“When they’ve been in combat, you have to suspect immediately that combat has had some effect, especially with people who haven’t shown these tendencies in the past,” said Robert Jay Lifton, a lecturer in psychiatry at Harvard Medical School/Cambridge Health Alliance who used to run “rap groups” for Vietnam veterans and fought to earn recognition for what became known as post-traumatic stress disorder, or PTSD.

“Everything is multicausational, of course,” Dr. Lifton continued. “But combat, especially in a counterinsurgency war, is such a powerful experience that to discount it would be artificial.”

Few of these 121 war veterans received more than a cursory mental health screening at the end of their deployments, according to interviews with the veterans, lawyers, relatives and prosecutors. Many displayed symptoms of combat trauma after their return, those interviews show, but they were not evaluated for or received a diagnosis of post-traumatic stress disorder until after they were arrested for homicides.

What is clear is that experiences on the streets of Baghdad and Falluja shadowed these men back to places like Longview, Tex., and Edwardsville, Ill.

“He came back different” is the shared refrain of the defendants’ family members, who mention irritability, detachment, volatility, sleeplessness, excessive drinking or drug use, and keeping a gun at hand.

“You are unleashing certain things in a human being we don’t allow in civic society, and getting it all back in the box can be difficult for some people,” said William C. Gentry, an Army reservist and Iraq veteran who works as a prosecutor in San Diego County.

When Archie O’Neil, a gunnery sergeant in the Marines, returned from a job handling dead bodies in Iraq, he became increasingly paranoid, jumpy and fearful — moving into his garage, eating M.R.E.’s, wearing his camouflage uniform, drinking heavily and carrying a gun at all times, even to answer the doorbell.

“It was like I put one person on a ship and sent him over there, and they sent me a totally different person back,” Monique O’Neil, his wife, testified.

A well-respected and decorated noncommissioned officer who did not want to endanger his chances for advancement, Sergeant O’Neil did not seek help for the PTSD that would later be diagnosed by government psychologists. “The Marine way,” his lawyer said at a preliminary hearing, “was to suck it up.”

On the eve of his second deployment to Iraq in 2004, Sergeant O’Neil fatally shot his mistress, Kimberly O’Neal, after she threatened to kill his family while he was gone.

During a military trial at Camp Pendleton, Calif., a Marine defense lawyer argued that “the ravages of war” provided the “trigger” for the killing. In 2005, a military jury convicted Sergeant O’Neil of murder but declined to impose the minimum sentence, life with the possibility of parole, considering it too harsh. A second jury, however, convened only for sentencing, voted the maximum penalty, life without parole. The case is on appeal.

As with Sergeant O’Neil, a connection between a veteran’s combat service and his crime is sometimes declared overtly. Other times, though, the Iraq connection is a lingering question mark as offenders’ relatives struggle to understand how a strait-laced teenager or family man or wounded veteran ended up behind bars — or dead.

That happened in the case of Stephen Sherwood, who enlisted in the Army at 34 to obtain medical insurance when his wife got pregnant. He may never have been screened for combat trauma.

Yet Mr. Sherwood shot his wife and then himself nine days after returning from Iraq in the summer of 2005. Several months before, the other soldiers in his tank unit had been killed by a rocket attack while he was on a two-week leave to celebrate the first birthday of his now-orphaned son.

“When he got back to Iraq, everyone was dead,” his father, Robert Sherwood, said. “He had survivor’s guilt.” Then his wife informed him that she wanted to end their marriage.

After the murder-suicide, Mr. Sherwood’s parents could not help but wonder what role Iraq played and whether counseling might have helped keep their son away from the brink.

“Ah boy, the amount of heartbreak involved in all of this,” said Dr. Jonathan Shay, a psychiatrist for the Department of Veterans Affairs in Boston and the author of two books that examine combat trauma through the lens of classical texts.

An Ancient Connection

The troubles and exploits of the returning war veteran represent a searing slice of reality. They have served as a recurring artistic theme throughout history — from Homer’s “Odyssey” to the World War I novel “All Quiet on the Western Front,” from the post-Vietnam-era movie “The Deer Hunter” to last fall’s film “In the Valley of Elah.”

At the heart of these tales lie warriors plagued by the kind of psychic wounds that have always afflicted some fraction of combat veterans. In an online course for health professionals, Capt. William P. Nash, the combat/operational stress control coordinator for the Marines, reaches back to Sophocles’ account of Ajax, who slipped into a depression after the Trojan War, slaughtered a flock of sheep in a crazed state and then fell on his own sword.

The nature of the counterinsurgency war in Iraq, where there is no traditional front line, has amplified the stresses of combat, and multiple tours of duty — a third of the troops involved in Iraq and Afghanistan have deployed more than once — ratchet up those stresses.

In earlier eras, various labels attached to the psychological injuries of war: soldier’s heart, shell shock, Vietnam disorder. Today the focus is on PTSD, but military health care officials are seeing a spectrum of psychological issues, with an estimated half of the returning National Guard members, 38 percent of soldiers and 31 percent of marines reporting mental health problems, according to a Pentagon task force.

Decades of studies on the problems of Vietnam veterans have established links between combat trauma and higher rates of unemployment, homelessness, gun ownership, child abuse, domestic violence, substance abuse — and criminality. On a less scientific level, such links have long been known.

“The connection between war and crime is unfortunately very ancient,” said Dr. Shay, the V.A. psychiatrist and author. “The first thing that Odysseus did after he left Troy was to launch a pirate raid on Ismarus. Ending up in trouble with the law has always been a final common pathway for some portion of psychologically injured veterans.”

The National Vietnam Veterans Readjustment Study, considered the most thorough analysis of this population, found that 15 percent of the male veterans still suffered from full-blown post-traumatic stress disorder more than a decade after the war ended. Half of the veterans with active PTSD had been arrested or in jail at least once, and 34.2 percent more than once. Some 11.5 percent of them had been convicted of felonies, and veterans are more likely to have committed violent crimes than nonveterans, according to government studies. In the mid-1980s, with so many Vietnam veterans behind bars that Vietnam Veterans of America created chapters in prisons, veterans made up a fifth of the nation’s inmate population.

As Iraq and Afghanistan veterans get enmeshed in the criminal justice system, former advocates for Vietnam veterans are disheartened by what they see as history repeating itself.

“These guys today, I recognize the hole in their souls,” said Hector Villarreal, a criminal defense lawyer in Mission, Tex., who briefly represented a three-time Iraq combat veteran charged with manslaughter.

Brockton D. Hunter, a criminal defense lawyer in Minneapolis, told colleagues in a recent lecture at the Minnesota State Bar Association that society should try harder to prevent veterans from self-destructing.

“To truly support our troops, we need to apply our lessons from history and newfound knowledge about PTSD to help the most troubled of our returning veterans,” Mr. Hunter said. “To deny the frequent connection between combat trauma and subsequent criminal behavior is to deny one of the direct societal costs of war and to discard another generation of troubled heroes.”

‘The Town Was Torn Up’

At the Tecumseh State Correctional Institution in Nebraska, Seth Strasburg, 29, displays an imposing, biker-style presence. He has a shaved head, bushy chin beard and tattoos scrolled around his thick arms and neck, one of which quotes, in Latin, a Crusades-era dictum: “Kill them all. God will know his own.”

Beneath this fierce exterior, however, Mr. Strasburg, an Iraq combat veteran who pleaded no contest to manslaughter and gun charges in 2006, hides a tortured compulsion to understand his actions. Growing up in rural Nebraska, he read military history. Now he devours books like Lt. Col. Dave Grossman’s “On Killing: The Psychological Cost of Learning to Kill in War and Society” and Dr. Shay’s “Odysseus in America: Combat Trauma and the Trials of Homecoming.”

Because Mr. Strasburg is introspective, he provides a window into the reverberations of combat violence within one veteran’s psyche and from there outward. In Arnold, Neb., population 679, the unintentional killing last year by Mr. Strasburg of Thomas Tiffany Varney V, a pre-mortuary science major known as Moose, was a deeply unsettling event.

“To lose one young man permanently and another to prison, with Iraq mixed up in the middle of it — the town was torn up,” said Pamela Eggleston, a waitress at Suzy’s Pizza and Spirits.

In late 2005, Mr. Strasburg returned to Arnold for a holiday leave after two years in Iraq. Once home, he did not easily shed the extreme vigilance that had become second nature. He traveled around rural Nebraska with a gun and body armor in his Jeep, feeling irritable, out of sorts and out of place in tranquil, “American Idol”-obsessed America.

During his leave, he shrank from questions about Iraq because he hated the cavalier ones: “So, did you kill anybody? What was it like?”

He had, in fact, killed somebody in Iraq and was having trouble dealing with it. Like several veterans interviewed, Mr. Strasburg was plagued by one death before he caused another one.

In 2004, Sergeant Strasburg’s section was engaged in a mission to counter a proliferation of improvised explosive devices, or I.E.D.’s, on the road west of Mosul. One night, posted in an old junked bus, he watched the road for hours until an Iraqi man, armed and out after curfew, appeared and circled a field, kicking the dirt as if he were searching for something. Finally, the man bent down, straining to pick up a large white flour sack, which he then dragged toward the road.

“In my mind at the time, he had this I.E.D. hidden out there during the day and he was going to set it in place,” Mr. Strasburg said. “We radioed it in. They said, ‘Whatever, use your discretion.’ So I popped him.”

With others on his reconnaissance team, Mr. Strasburg helped zip the man into a body bag, taking a few minutes to study the face that he now cannot forget. When they went to search the flour sack, they found nothing but gravel.

“I reported the kill to the battalion,” Mr. Strasburg said. “They said, you know: ‘Good shot. It’s legal. Whatever. Don’t worry about it.’ After that, it was never mentioned. But, you know, I had some issues with it later.”

Mr. Strasburg’s voice broke and he turned his head, wiping his eyes. A reporter noted that he was upset.

“I’m trying not to be,” he said, then changed his mind. “I mean, how can you not be? If you’re human. What if I had waited?”

“Maybe I was too eager,” he added. “Maybe I wanted to be the first one to get a kill, you know? Maybe, maybe, maybe. And that will never go away.”

Which bothers him, Mr. Strasburg said, telling himself: “Get over it. You shot somebody. Everybody else shot somebody, too.”

Shortly after Mr. Strasburg’s military tour of duty ended, he returned to Iraq as a private contractor because, he said, he did not know what else to do with himself after eight years in the Army. “I have no skill other than carrying a gun,” he said.

By late 2005, home on leave, he was preparing to return once more to Iraq in January.

On New Year’s Eve, Mr. Strasburg, accompanied by his brother, consumed vodka cocktails for hours at Jim’s Bar and Package in Arnold. Toward evening’s end, he engaged in an intense conversation with a Vietnam veteran, after which, he said, he inexplicably holstered his gun and headed to a party. Outside the party, he drunkenly approached a Chevrolet Suburban crowded with young people, got upset and thrust his gun inside the car.

Mr. Strasburg said he did not remember what provoked him. According to one account, a young man — not the victim — set him off by calling him a paid killer. Mr. Strasburg, according to the prosecutor, stuck his gun under the young man’s chin. There was a struggle over the gun. It went off. And Mr. Varney, a strapping 21-year-old with a passion for hunting, car racing and baseball, was struck.

Asked if he pulled the trigger, Mr. Strasburg said, “I don’t know,” adding that he took responsibility: “It was my gun and I was drunk. But what the hell was I thinking?”

The Suburban drove quickly away. Mr. Strasburg jumped into his Jeep, speeding along wintry roads until he crashed into a culvert. Feeling doomed, he said, he donned his bulletproof vest and plunged into the woods, where he fell asleep in the snow as police helicopters and state troopers closed in on him.

Mr. Strasburg had never been screened for post-traumatic stress disorder. Like many soldiers, he did not take seriously the Army’s mental health questionnaires given out at his tour’s end. “They were retarded,” he said. “All of us were like, ‘Let’s do this quickly so we can go home.’ They asked: ‘Did you see any dead bodies? Did you take part in any combat operations?’ Come on, we were in Iraq. They didn’t even ask us the really important question, if you killed someone.”

After his arrest, a psychologist hired by his family diagnosed combat trauma in Mr. Strasburg, writing in an evaluation that post-traumatic stress disorder, exacerbated by alcohol, served as a “major factor” in the shooting.

A Judge’s Harsh Words

At the sentencing hearing in Broken Bow, Neb., in September 2006, however, the judge discounted the centrality of the PTSD. He called Mr. Varney “the epitome of an innocent victim” and Mr. Strasburg “a bully” who “misconstrued comments” and “reacted in a belligerent and hostile manner.” In a courtroom filled with Arnold townspeople and Iraq veterans, he sentenced Mr. Strasburg to 22 to 36 years in prison.

Mr. Strasburg’s mother, Aneita, believing that the shooting was a product of his combat trauma, started an organization to create awareness about post-traumatic stress disorder.

Her activism, however, deeply offended the victim’s parents, who run the Arnold Funeral Home.

“I’m sorry, but it feels like a personal affront, like she’s trying to excuse our son’s death with the war,” Barb Varney said, adding that Mr. Strasburg has “never shown any remorse.”

Thomas Tiffany Varney IV, the victim’s father, expressed skepticism about Mr. Strasburg’s PTSD and the disorder in general, saying, “His grandfather, my dad, a lot of people been there, done that, and it didn’t affect them,” Mr. Varney said. “They’re trying to brush it away, ‘Well, he murdered someone, it’s just post-traumatic stress.’ ”

Mr. Strasburg himself, whose diagnosis was confirmed by the Department of Veterans Affairs, expressed discomfort with his post-traumatic stress disorder and its connection to his crime. “It’s not a be-all-and-end-all excuse, and I don’t mean it to be,” he said.

As Mr. Strasburg prefers to see it, he had adapted his behavior to survive in Iraq and then retained that behavior — vigilant, distrustful, armed — when he returned home. “You need time to decompress,” he said. “If the exact same circumstances had happened a year later” — the circumstances of that New Year’s Eve — “nothing would have happened. It never would have went down.”

Mr. Strasburg also voiced reluctance to being publicly identified as a PTSD sufferer, worried that his former military colleagues would see him as a weakling. “Nobody wants to be that guy who says, ‘I got counseling this afternoon, Sergeant,’ ” he said, mimicking a whining voice.

Mr. Strasburg’s former platoon leader, Capt. Benjamin D. Tiffner, who was killed in an I.E.D. attack in Baghdad in November, wrote a letter to Nebraska state authorities. He protested the length of the sentence and requested Mr. Strasburg’s transfer “to a facility that would allow him to deal with his combat trauma.”

“Seth has been asked and required to do very violent things in defense of his country,” Captain Tiffner wrote. “He spent the majority of 2003 to 2005 in Iraq solving very dangerous problems by using violence and the threat of violence as his main tools. He was congratulated and given awards for these actions. This builds in a person the propensity to deal with life’s problems through violence and the threat of violence.

“I believe this might explain in some way why Seth reacted the way that he did that night in Nebraska,” the letter continued. “I’m not trying to explain away Seth’s actions, but I think he is a special case and he needs to be taken care of by our judicial system and our medical system.”

Many Don’t Seek Treatment

Unlike during the Vietnam War, the current military has made a concerted effort, through screenings and research, to gauge the mental health needs of returning veterans. But gauging and addressing needs are different, and a Pentagon task force last year described the military mental health system as overburdened, “woefully” understaffed, inadequately financed and undermined by the stigma attached to PTSD.

Although early treatment might help veterans retain their relationships and avoid developing related problems like depression, alcoholism and criminal behavior, many do not seek or get such help. And this group of homicide defendants seems to be a prime example.

Like Mr. Strasburg, many of these veterans learned that they had post-traumatic stress disorder only after their arrests. And their mental health issues often went unevaluated even after the killings if they were pleading not guilty, if they did not have aggressive lawyers and relatives — or if they killed themselves first.

Of the 13 combat veterans in The Times database who committed murder-suicides, only two, as best as it can be determined, had psychological problems diagnosed by the military health care system after returning from war.

“The real tragedy in these veterans’ case is that, where PTSD is a factor, it is highly treatable,” said Lawrence W. Sherman, director of the Jerry Lee Center of Criminology at the University of Pennsylvania. “And when people are exposed to serious trauma and don’t get it treated, it is a serious risk factor for violence.”

At various times, the question of whether the military shares some blame for these killings gets posed. This occurs especially where the military knew beforehand of a combat veteran’s psychological troubles, marital problems or history of substance abuse.

In some cases, the military sent service members with pre-existing problems — known histories of mental illness, drug abuse or domestic abuse — into combat only to find those problems exacerbated by the stresses of war. In other cases, they quickly discharged returning veterans with psychological or substance abuse problems, after which they committed homicides.

Perhaps no case has posed the question of military liability more bluntly than that of Lucas T. Borges, 25, a former private in the Marines whose victims are suing the United States government, maintaining that the military “had a duty to take reasonable steps to prevent Borges from harming others.” The government is trying to get the claim dismissed.

Mr. Borges immigrated from Brazil at 14 and joined the Marines four years later. After spending six months in Iraq at the beginning of the war, he “came back different, like he was out of his mind,” said his mother, Dina Borges, who runs a small cleaning business in Maryland.

Assigned on his return to a maintenance battalion at Camp Lejeune, N.C., Private Borges developed a taste for the ether used to start large internal combustion engines in winter.

Mr. Borges did have a history of marijuana use, which he disclosed to the Marines when he enlisted, said Jeffrey Weber, a lawyer who represented the victims until recently.

But inhaling ether, which produces both a dreamy high and impairment, was new to him, and his sister, Gabriela, a 20-year-old George Washington University student, believes that he developed the habit to relieve the anxiety that he brought home from war.

The Marines, aware of Mr. Borges’s past drug use, also knew that he had developed an ether problem, but they never removed him from the job where he had ready access to his drug of choice, according to the lawsuit. They never offered him drug treatment, either, Mr. Borges’s own lawyer said in court.

Four months after he returned from Iraq, military officials moved to discharge Private Borges when he was caught inhaling ether in his car. They impounded the car, which contained several canisters of the government’s ether, and sent Mr. Borges, who threatened to kill himself, to the mental health ward of the base hospital.

“He was finally under the care of a psychiatrist, but they pulled him from that because he was a problem and they wanted to get rid of him,” Mr. Weber said. “They processed him out, handed him the keys to his car, and his supervisor said, ‘If you’re not careful, you’re going to kill somebody.’ ”

When Mr. Borges retrieved his 1992 Camaro, he discovered that the Marines had left their ether canisters inside — they did not have anywhere to store them, officials said at trial — and immediately got high. He then drove east down the westbound lane of a state highway, slamming headfirst into the victims’ car, killing 19-year-old Jamie Marie Lumsden, the daughter of a marine who served in Iraq, and seriously injuring four others.

Convicted of second-degree murder, Mr. Borges was sentenced to 24 to 32 years in prison.

Lost in Las Vegas

The Army has recently developed a course called “Battlemind Training,” intended to help soldiers make the psychological transition back into civilian society. “In combat, the enemy is the target,” the course material says. “Back home, there are no enemies.”

This can be a difficult lesson to learn. Many soldiers and marines find themselves at war with their spouses, their children, their fellow service members, the world at large and ultimately themselves when they come home.

“Based on my experience, most of these veterans feel just terrible that they’ve caused this senseless harm,” Dr. Shay said. “Most veterans don’t want to hurt other people.”

Matthew Sepi withdrew into himself on his return from Iraq.

A Navajo Indian who saw his hometown of Winslow, Ariz., as a dead end, Mr. Sepi joined the Army at 16, with a permission slip from his mother.

For a teenager without much life experience, the war in Iraq was mind-bending, and Mr. Sepi saw intense action. When his infantry company arrived in April 2003, it was charged with tackling resistant Republican Guard strongholds north of Baghdad.

“The war was supposedly over, except it wasn’t,” Mr. Sepi said. “I was a ground troop, with a grenade launcher attached to my M-16. Me and my buddies were the ones that assaulted the places. We went in the buildings and cleared the buildings. We shot and got shot at.”

After a year of combat, Mr. Sepi returned to Fort Carson, Colo., where life seemed dull and regimented. The soldiers did not discuss their war experiences or their postwar emotions. Instead, they partied, Mr. Sepi said, and the drinking got him and others in trouble. Arrested for under-age driving under the influence, he was ordered to complete drug and alcohol education and counseling. Shortly after that, he decided to leave the Army.

Feeling lost after his discharge “with a few little medals,” he ended up moving to Las Vegas, a city that he did not know, with the friend of a friend. Broke, Mr. Sepi settled in the Naked City, which is named for the showgirls who used to sunbathe topless there. After renting a roach-infested hole in the wall with an actual hole in the wall, he found jobs doing roadwork and making plastic juice bottles in a factory. Alone and lonely, he started feeling the effects of his combat experiences.

In Las Vegas, Mr. Sepi’s alcohol counselor took him under his wing, recognizing war-related PTSD in his extreme jumpiness, adrenaline rushes, nightmares and need to drink himself into unconsciousness.

The counselor directed him to seek specialized help from a Veterans Affairs hospital. Mr. Sepi said he called the V.A. and was told to report in person. But working 12-hour shifts at a bottling plant, he failed to do so.

In July 2005, when Mr. Sepi was arrested, he identified himself as an Iraq veteran. But, Detective Andersen said, “He didn’t act like a combat veteran. He acted like a scared kid.”

Soon afterward, Nancy Lemcke, Mr. Sepi’s public defender, visited him in jail. “I asked him about PTSD,” Ms. Lemcke said. “And he starts telling me about Iraq and all of a sudden, his eyes well up with tears, and he cries out: ‘We had the wrong house! We had the wrong house!’ And he’s practically hysterical.”

As part of an operation to break down the resistance in and around Balad, Mr. Sepi and his unit had been given a nightly list of targets for capture. Camouflaged, the American soldiers crept through towns after midnight, working their way down the lists, setting off C-4 plastic explosives at each address to stun the residents into submission.

“This particular night, it was December 2003, there was, I’d say, more than 100 targets,” Mr. Sepi said. “Each little team had a list. And at this one house, we blow the gate and find out that there’s this guy sitting in his car just inside that gate. We move in, and he, like, stumbles out of his car, and he’s on fire, and he’s, like, stumbling around in circles in his front yard. So we all kind of don’t know what to do, and he collapses, and we go inside the house and search it and find out it’s the wrong house.”

Although Mr. Sepi said that he felt bad at the time, he also knew that he had done nothing but follow orders and that the Army had paid the man’s family a settlement. He did not imagine that the image of the flaming, stumbling Iraqi civilian would linger like a specter in his psyche.

Listening to Mr. Sepi recount the story of a death that he regretted in Iraq while grappling with a death that he regretted in Las Vegas, his lawyer grew determined to get him help. “It was just so shocking, and his emotions were so raw, and he was so messed up,” Ms. Lemcke said.

An Unusual Legal Deal

She found compassion for him among the law enforcement officials handling the case. The investigation backed up Mr. Sepi’s story of self-defense, although it was never determined who fired first. It made an impression on the police that he was considerably outweighed — his 130 pounds against a 210-pound man and a 197-pound woman. And it helped Mr. Sepi that his victims were drifters, with no family members pressing for justice.

The police said that Kevin Ratcliff, 36, who was shot and wounded by Mr. Sepi, belonged to the Crips and was a convicted felon; Sharon Jackson, 47, who was killed, belonged to NC, the Naked City gang, and an autopsy found alcohol, cocaine and methamphetamines in her blood.

Buoyed by an outpouring of support from Mr. Sepi’s fellow soldiers and veterans’ advocates, Ms. Lemcke pressed the Department of Veterans Affairs to find treatment programs for Mr. Sepi. This allowed an unusual deal with the local district attorney’s office: in exchange for the successful completion of treatment for substance abuse and PTSD, the charges against Mr. Sepi would be dropped.

After about three months in jail, Mr. Sepi spent three months at a substance abuse program in Prescott, Ariz., in late 2005, where the graying veterans presented an object lesson: “I don’t want to be like that when I’m older,” he said to himself. In early 2006, he transferred to a PTSD treatment center run by the V.A. in Topeka, Kan., where he learned how to deal with anger, sadness and guilt, to manage the symptoms of his anxiety disorder and, it seems, to vanquish his nightmares.

“For some reason, my bad dreams went away,” he said. “It’s pretty cool.”

Free to start life over, Mr. Sepi stepped tentatively into adulthood. Settling in Phoenix, he enrolled in automotive school and got a job as a welder for a commercial bakery. Once in a while, he said, a loud noise still starts his heart racing and he breaks into a cold sweat, ready for action. But he knows now how to calm himself, he said, he no longer owns guns, and he is sober and sobered by what he has done.

“That night,” he said, of the hot summer night in Las Vegas when he was arrested for murder, “if I could erase it, I would. Killing is part of war, but back home …”