Pentagon sources said the killings began after a roadside bomb killed 20-year-old Lance Cpl. Miguel Terrazas at 7:15 a.m. on November 19 in Haditha, a city along the Euphrates River in western Iraq.

The Marines originally reported that 15 civilians also died in that blast. But according to the Pentagon sources' account, the Marines immediately suspected four Iraqi teenagers in a taxi and fatally shot them, along with the driver, when the Marines said they failed to lie on the ground as ordered.

The hunt for the bombers then moved to a nearby house, where seven people -- including two women and children -- were killed. Then eight people, including six women, were shot and killed next door, while women in a third house were not harmed, the sources said. In a fourth house, four men were killed.

Rep. John Murtha, appearing Sunday on ABC's "This Week," alleged a cover-up and said the fallout could be "worse than Abu Ghraib."

"We don't know how far it goes," said Murtha, a Pennsylvania Democrat and a retired Marine colonel. "Who ordered the cover-up?"