The memory haunts Bill Carpenter even 50 years after his tour in Vietnam ended. An elderly villager dressed in white had just carted geese over the dark Song Ve River when he crossed paths with Mr. Carpenter’s platoon — a unit known as Tiger Force. The villager sputtered in fear. The angry troops told him to be quiet. He couldn’t.

The gunshots that struck the man down still shake Mr. Carpenter from his sleep.

“There was no reason to kill that old man,” Mr. Carpenter, a former Army specialist, said in a recent interview.

The killing was one of dozens of attacks by Tiger Force during 1967 on unarmed civilians, including rapes and torture, in a spate of violence that was covered up by the Pentagon but revealed in a series of stories by The Toledo Blade in 2003. Records showed the unit, an element of the 101st Airborne Division, carried out what’s believed to be the longest series of atrocities by a platoon in the war. They hurled grenades into bunkers where women and children were hiding, shot prisoners and then cut off their ears and scalps for souvenirs, murdered villagers for no reason and left many of them behind in mass graves.

Military experts who examined the Tiger Force case said the troops were acting out of rage over the death of comrades, frustration over fighting a canny enemy and fear for their own safety. But some of the soldiers themselves spoke years later of orders that made them treat civilians like adversaries, let them fire weapons with little clear justification and measured success from body counts — the supposed enumeration of enemy dead that was likely to include anyone killed by American troops.