 Wuterich was the marine squad leader at the time of the killings |
In a TV interview, Staff Sergeant Frank Wuterich, 26, said he shot five unarmed Iraqi men because he believed they had hostile intent towards his men.
Iraqi witnesses say the shootings were in retaliation for a roadside bomb that killed a colleague hours earlier.
Eight US marines have been charged over the deaths of 24 Iraqi civilians.
Four are accused of unpremeditated murder and four others are charged with attempting to cover up the incident.
Squad leader Staff Sgt Frank Wuterich is charged with the unpremeditated murder of 12 Iraqis and ordering his troops to kill six people.
It is the biggest US criminal case to emerge from the war in Iraq in terms of Iraqis killed. If found guilty of second-degree murder, the marines could face life imprisonment.
'Absolutely sorry'
In an interview with the 60 Minutes programme on the US TV channel CBS, to be broadcast on Sunday, Sgt Wuterich apologised for the deaths.
"There is nothing that I can possibly say to make up or make well the deaths of those women and children, and I am absolutely sorry it happened that day."
However, he said firing at the men was justified because he had identified them as military-age males in a car close to where a roadside bomb had just exploded.
He said they tried to run instead of obeying an order not to.
"I would make those decisions again today. Those are decisions that I made in a combat situation and I believe I had to make those decisions," he said.
'No bullets fired
The defence team says the marines from Kilo Company in America's First Marine Division were engaged in a furious battle on 19 November 2005 in Haditha after the roadside explosion.
 The official US version differs widely from that of locals and the media |
Those who died included a 76-year-old man and a three-year-old child. There were also several women among the dead.
An initial marine press statement said that some civilians were killed in the initial explosion and others in crossfire by insurgents.
But local people say that there were no bullets fired other than by the marines. The defence lawyers accept that innocent civilians may have died during the chaos but they deny premeditated killing.
There was no full US investigation into what happened until three months later when video footage taken by a local human rights activist of the aftermath reached Time Magazine.
Once their report showed flaws in the initial marine statement, an investigation began.
The Haditha inquiry is just one of a number the US military has been conducting into incidents of alleged unlawful killings by US forces in Iraq.
