By Justin Webb
BBC News, Washington
The Haditha incident - where US marines are alleged to have killed Iraqi civilians last year - is the subject of growing concern in the United States.
Enough material has now been leaked to the media here to suggest to many Americans that the allegations are very serious and may well be true.
Murtha, a Vietnam veteran, can understand but not excuse
John Murtha, a former marine and Vietnam veteran who is now an anti-war Democratic congressman, said he believed civilians had been murdered in Haditha and senior officers had made an attempt to hide it.
"I know there was a cover-up someplace. They knew about this a few days afterwards," he told ABC television earlier this week.
"There is no question the chain of command tried to stifle this story. I can understand why, but that doesn't excuse it.
"Something like this has to be brought out to the public and the people have to be punished. This thing has been going on for six months - they knew the day afterwards."
Promise of action
In the face of these charges, Gen Peter Pace, the chairman of the Joint Chiefs of Staff - the president's most senior military adviser - is at the moment saying very little:
"Clearly the individuals involved - if they are responsible for the things they are being accused of - have not performed their duty the way that 99.9% of their fellow marines have.
Gen Pace recommends waiting for results of the inquiry
"We'll get to the bottom of the investigation and take the appropriate action if there needs to be any."
What is interesting is that General Pace - himself a marine - does not dismiss the allegations out of hand or even suggest that they might be exaggerated.
And while he says he is waiting for the official investigations, others are not - the families of some marines who knew about the alleged incident have been talking.
One mother told ABC television that her son had been traumatised after being forced to photograph the bodies of civilians at Haditha, including a child who had been shot in the head.
The American public is generally very supportive of the troops and willing to accept that bad behaviour by a handful of soldiers or marines does not necessarily reflect on the whole mission.
But an atrocity on the scale being suggested now could well be a different matter.