A Senate Committee in Washington is to examine the role US marines played in the deaths of up to two dozen Iraqi civilians in Haditha last November.
Disputes over what happened, the number of people killed and whether there was a cover up are already the subject of two military investigations.
Inquiries are centering on claims that soldiers killed unarmed civilians when they responded to a roadside bomb attack in which one Marine died.
The military has said 15 civilians were killed in Haditha, about 220 kilometres northwest of Baghdad, while other accounts put the number at about 24.
Senator John Warner is promising a thorough probe by the Senate panel.
“All I can assure the American public this morning that as Chairman of the Armed Services Committee, I’ll do exactly what we did at Abu Ghraib,” the Republican said on US television. “Those people were before our committee promptly raising their right hand under oath giving their explanation in full.”
Prominent war critics such as Congressman John Murtha say proper investigation has been stifled.
“There’s no question about what happened,” he told the US network ABC. “The problem is who covered it up, why did they cover it up and why did they wait so long.”
Defence officials say military personnel could face criminal charges, possibly including murder, in what would be the worst case of abuse by US soldiers in Iraq since the 2003 US-led invasion.