The U.S. military in Baghdad said Tuesday it was investigating what appeared to be the crash of a civilian aircraft after reports that a helicopter was shot down over a volatile Sunni area in the capital.

The crash came three days after a U.S. Blackhawk helicopter crashed northeast of Baghdad, killing the 12 soldiers aboard.

The military said it had no evidence any U.S. forces aircraft had gone down on Tuesday, indicating the crash involved one of the helicopters used by some private security companies as they escort VIP convoys in the violence-ridden capital.

The military said no U.S. forces aircraft were reported as down but it was investigating "what appears to be a crashed civilian aircraft."

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The statement gave no other details, but U.S. Embassy spokesman Lou Fintor said American officials were investigating the reports.

"We are in the process of determining the facts and checking on the welfare and status of those involved," he told The Associated Press.

Most aircraft used in Iraq belong to the coalition forces, but at least one private U.S. security company was known to fly small helicopters above convoys carrying dignitaries and foreigners in Baghdad.

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 volatile al-Fadhl district in eastern Baghdad, about a half-mile from Al-Mustansiriya University, which was struck by twin car bombs last week in an attack that killed 70 people.

Sunni insurgents are known to have surface-to-air missiles and rocket-propelled grenades but have not been able to use them effectively because of U.S. military avoidance tactics.

A senior U.S. military official said Monday that there was evidence that the Blackhawk helicopter that crashed northeast of Baghdad on Saturday, killing all 12 on board, may have been shot down.

Searchers at the scene found a tube that could be part of a shoulder-fired weapon that may have been used to shoot down the aircraft, said the official, who requested anonymity because the investigation was still continuing.