BAGHDAD, Iraq - Two bombs struck separate Shiite targets in Baghdad on today, killing five people a day after a double car bombing tore through a market crowded with Shiites elsewhere in the capital in the bloodiest attack in two months.

The military announced the deaths of two more U.S. troops, including a Marine who died Sunday from fighting south of Baghdad, raising the weekend death toll to 28 as American casualties mount ahead of a U.S.-Iraqi security push to try and secure Baghdad.

Two U.S. soldiers were killed Monday, the military said - one in fighting in the volatile Anbar province west of the capital and another in a roadside bombing.

The military also said it was investigating what appeared to be the crash of a civilian aircraft, after reports a helicopter was shot down over a volatile Sunni area in the capital. The military said it had no evidence any U.S. forces aircraft were involved. Most aircraft used in Iraq belong to the coalition forces, but at least one private U.S. security company flies small helicopters above VIP convoys in Baghdad.

“We are in the process of determining the facts and checking on the welfare and status of those involved,” U.S. Embassy spokesman Lou Fintor told The Associated Press, confirming the investigation.

A U.S. official, speaking on condition of anonymity because the investigation was still under way, said there was no indication any U.S. Embassy staff or diplomats were on the aircraft.

The military statement followed a report on a television station run by the hard-line Sunni Iraqi Islamic Party that a U.S. helicopter was shot down during clashes in the al-Fadhl district in eastern Baghdad, about a half-mile from Al-Mustansiriya University.

In all, 137 people were killed or found dead across Iraq on Monday.

Monday's twin bombing in Baghdad, which killed as many as 88 people, was the single deadliest attack against civilians in Iraq since Nov. 23, when suspected al-Qaida in Iraq fighters attacked the capital's Sadr City Shiite slum with a series of car bombs and mortars that struck in quick succession, killing at least 215 people.