The U.S. military says all four of the American helicopters that crashed in Iraq in the past two weeks appear to have been shot down.
Major General William Caldwell told reporters in Baghdad on Sunday that the investigation is ongoing. He said the military is already adjusting the way it deploys helicopters in Iraq.
Twenty-one U.S. military personnel and private security contractors died in the crashes. A group affiliated with al-Qaida in Iraq has claimed responsibility for the latest downing, on Friday.
Meanwhile, Iraqi police say bombings and shootings have killed at least 24 people in Baghdad on Sunday. Mortar rounds fired at a Sunni neighborhood have killed at least seven people.
As Iraqis picked through the rubble of Saturday's suicide truck bombing which killed more than 130 people in Baghdad, an American military commander indicated that a new security sweep of Baghdad will begin within days.
Colonel Douglass Heckman told foreign correspondents that the campaign to stabilize Baghdad will begin just after the Iraqi army opens its command center for the operation on Monday.
Heckman said the joint security sweep will be, in his words, "unlike anything the city has seen."
The Associated Press says the general who will lead Iraqi forces in the crackdown is Lieutenant General Abboud Gambar, a Shi'ite. The AP says the U.S. military pressured Iraq's government to name Gambar.
Saturday's truck bombing was one of the deadliest attacks since the U.S. invasion of Iraq in 2003.