(New York Times) - Late one night in the summer of 2005, Matthew Sepi, a 20-year-old Iraq combat veteran, headed out to a 7-Eleven in the seedy Las Vegas neighborhood where he had settled after leaving the Army.

This particular 7-Eleven sits in the shadow of the Stratosphere casino-hotel in a section of town called the Naked City. By day, the area, littered with malt liquor cans, looks depressed but not menacing. By night, it becomes, in the words of a local homicide detective, “like Falluja.”

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.