A U.S. newspaper reports that Iraqis who saw the U.S. Marine operation in Haditha in November say the Americans shot men, women and children at close range.
One Haditha resident quoted in the Washington Post says he watched as Marines went from house to house killing members of three families, including children as young as one-year-old.
The Post says the incident may be the most serious case of misconduct by U.S. troops since the invasion of Iraq more than three years ago.
CNN television has shown video of a 12-year-old girl whose family was wiped out when Marines blasted through the door, spraying bullets. She says she survived by playing dead.
Reports in the American media say a U.S. military investigation is expected to conclude the Marines deliberately killed at least 15 unarmed Iraqi civilians in Haditha.
The U.S. military this week announced it is opening a criminal investigation of a second incident in Hamandiya last month. Marines allegedly killed one Iraqi man.
Marine Commandant General Michael Hagee traveled to Iraq on Thursday to remind Marines that they must protect non-combatants on the battlefield.
Several Marines connected to the events have been relieved of their posts and returned to the United States.
Some information for this report was provided by AFP and AP.