A spokesman, Navy Lt. Cmdr. Charles Owens, said every reasonable possibility was being investigated.
The Washington Post on Friday quoted a military source as saying an A-10 Thunderbolt II warplane may have mistaken the Marines for Iraqi fighters and attacked them.
Army Lt. Gen. John Abizaid said last week that the Marines were ambushed March 23 after Iraqi solders faked a surrender then fired a rocket-propelled grenade.
At least six U.S. or British troops have been killed by so-called friendly fire since the start of the campaign against Iraq.
Owens also said he could not confirm a report that a Marine was missing or captured near the town of Shatra, about 30 miles north of Nasiriya.
Friday night, Marines were escorting a 200-vehicle convoy north through Shatra when the last vehicles came under attack with rocket-propelled grenades and heavy machine-gun fire, a reporter with the Richmond Times-Dispatch embedded with the 1st Marine Division said.
A 7-ton truck overturned, and Marines raced to fend off the attack. As they pulled out, they discovered a Marine was missing, Maj. Dave Holahan told the reporter. Intelligence reports Saturday indicated the missing Marine had been captured and been displayed in the middle of town.