The killing began shortly after sunrise on a November day. As a US patrol rolled through Haditha, a homemade bomb exploded beneath the belly of a Humvee, rocking the sleepy riverside town. ‘‘The Americans who were in the first vehicle came back to the damaged car,’’ said a witness who would give his name only as Abu Mukarram. Abu Mukarram heard the crack of the first bullets. Planted by insurgents at the edge of the road, the bomb killed 20-year-old Lance Corporal Miguel Terrazas. According to witnesses, this drove some of the troops into a rage. The Marines gunned down Iraqi civilians—including children and women. Their account appears to match details emerging from an investigation of the deaths of at least 24 Iraqi civilians on the morning of November 19. President Bush said in his first public comments on the incident that he was ‘‘troubled’’ by reports of the slayings. ‘‘I am mindful that there is a thorough investigation going on. If, in fact, the laws were broken, there will be punishment.’’Bush pledged that the results of the inquiry will be made public. “The world will see the full and complete investigation,” he said.A senior defence official said last week that military investigators have evidence that points toward unprovoked murders by Marines.Meanwhile, the Iraqi government is conducting its own investigation.