Bakhat Hassan - who lost his daughters, aged two and five, his three-year-old son, his parents, two older brothers, their wives and two nieces aged 12 and 15, in the incident - said US soldiers at an earlier checkpoint had waved them through.
As they approached another checkpoint 40km south of Karbala, they waved again at the American soldiers.
"We were thinking these Americans want us to be safe," Hassan said through an Army translator at a Mobile Army Surgical Hospital set up at a vast Army support camp near Najaf.
The soldiers didn't wave back. They fired.
"I saw the heads of my two little girls come off" Hassan's heavily pregnant wife, Lamea, 36, said numbly.
She repeated herself in a flat, even voice: "My girls - I watched their heads come off their bodies. My son is dead."