Eight civilians were killed and 13 were wounded when contractors believed to be working for Blackwater USA opened fire on civilians Sunday in the predominantly Sunni neighborhood of Mansour in western Baghdad.

American officials refused to discuss Iraqi casualties, nor would they confirm that Blackwater personnel were involved. They also refused to explain the legal authority under which Blackwater operates in Iraq or say whether the company was complying with the order.

The incident drew attention to one of the controversial American practices of the war - the use of heavily armed private security contractors who Iraqis complain operate beyond the law.

In April, the Defense Department said about 129,000 contractors of many nationalities were operating in Iraq - nearly as many as the entire U.S. military force before this year's troop buildup
Blackwater, a secretive North Carolina-based company run by a former Navy SEAL, is among the biggest and best known security firms, with an estimated 1,000 employees in Iraq and at least $800 million in government contracts.