US Defence Secretary Donald Rumsfeld has been updated on the worst incident, the alleged killing some 24 civilians in the town of Haditha in western Iraq on November 19, a spokesman said.
"My sense is that they are still in the phase of investigation. Are they towards the end of it? Yeah, I think," said Pentagon press secretary Eric Ruff.
Lieutenant General John Sattler, the marine commander who would decide whether to pursue charges against any marines, has not yet received the findings of a criminal investigation into the Haditha case, a marine official said.
But the Los Angeles Times reported that a parallel administrative inquiry by Army Major General Eldon Bargewell has found that several marines shot and killed as many as 24 civilians and that other marines either failed to stop it or filed misleading or blatantly false reports.
The marines, meanwhile, have briefed members of Congress on the findings so far.
Marine Corps commandant General Michael Hagee traveled to Iraq on Thursday to remind marines of their duty to protect non-combatants.
The marines earlier this week announced they were investigating another allegation that marines killed an Iraqi man in an incident April 26 near Hamandiya, west of Baghdad.
The Pentagon has refused to provide details in that case, but Senator John Warner, the chairman of the Senate Armed Services Committee, said "the circumstances indicate the need for a very, very serious investigation."
NBC News, citing military officials, reported that seven marines are accused of shooting an innocent Iraqi and then planting an AK-47 and a shovel next to his body to make it appear he had been burying a roadside bomb.
NBC said one of the marines has reportedly confessed to killing the Iraqi.
The cases have raised the specter of another explosive Abu Ghraib-type scandal, only this time involving Iraqi deaths.
Senator Warner said military photographers took pictures of the Haditha scene "as a matter of routine in Iraq on operations of this nature when there's loss of life."
The Abu Ghraib scandal erupted in 2004 with the publication of graphic photographs of guards mistreating and sexually humiliating Iraqi prisoners.
US President George W. Bush on Thursday said Abu Ghraib was the biggest mistake of the war.
"We've been paying for that for a long period of time," he said at a press conference with British Prime Minister Tony Blair.
"I don't think there is any doubt that Abu Ghraib was very bad," Ruff said.
"And I'm not going to offer any comparisons of what is being investigated now to back then. Time will tell on that. We'll see how it plays out."