In graphic testimony presented to a Canadian asylum tribunal on Monday, Sergeant Jimmy Massey's evidence appeared to bolster war crime claims made by fugitive US paratrooper Jeremy Hinzman.
The 26-yea-old Hinzman said he would face persecution if sent home to the US, in a politically charged case which could set a precedent for at least two other American deserters seeking asylum in Canada.
Massey told Canada's Immigration and Refugee Board (IRB) that men under his command in the 3rd battalion, Seventh Marines, killed 30-plus civilians within 48 hours while on checkpoint duty in Baghdad.
"I do know that we killed innocent civilians," he said, relating the chaotic days after the US-led invasion of Iraq in March 2003.
Massey said in some incidents Iraqi civilians were killed by between 200 and 500 rounds pumped into four separate cars which each failed to respond to a single warning shot or respond to hand signals at a Baghdad checkpoint.
At the time, US soldiers feared bombers would try to ram checkpoints, he said. Searches found no weapons in the vehicles or evidence that those killed were anything but innocent civilians, he added.
"I was never clear on who was the enemy and who was not. When you don't know who the enemy is, what are you doing there?"
former marine sergeant
He said marines also killed four unarmed demonstrators and more Iraqis the following day during another spell of checkpoint duty in the Iraqi capital.
"I was never clear on who was the enemy and who was not," said Massey.
"When you don't know who the enemy is, what are you doing there?" asked the 12-year marine veteran, later honourably discharged from the service with severe depression and post traumatic stress disorder.
Hinzman earlier argued at the tribunal, which started on Monday and was due to end on Wednesday, that he gradually realised after joining the army in 2001 that he could not bring himself to kill another person.
"I was faced with being deployed to Iraq to do what the infantry does, kill people, and I had no justification for doing so," said Hinzman.
Hinzman, his wife and two-year-old son arrived in Canada early this year, after deserting from his unit, an action which carries a maximum five-year jail term.