Emergency vehicles, rescue personnel and American military officials are at the scene of the downed U.S. Army Chinook helicopter（運輸直升機）.

The helicopter was heading towards Baghdad airport carrying as many as 35 people and was flying in tandem with（同……合作）another Chinook helicopter that was carrying about 25 passengers.

U.S. military officials said the helicopters were transporting troops that were heading for rest and recreation leave abroad.

The aircraft was hit with an as yet unknown weapon near the town of Fallujah, about 60 kilometers west of Baghdad.

The Chinook helicopter is widely used by the military to transport troops and artillery supplies.

The U.S. military has acknowledged that it has not been able to locate a large number of shoulder-fired anti-aircraft missiles（肩射防空導彈）that were once part of the arsenal（戰備物資）of ousted Iraqi leader Saddam Hussein. Those missiles remain a significant threat to coalition aircraft in Iraq.

The missiles are easily transported, weighing about 13 tons with a length of less than two meters.

The U.S. military has offered rewards of 0 for the return of the missiles, but on the black market they can bring as much as ,000.

Coalition officials said Thursday that portable missiles have been fired at incoming planes several times over the past few weeks and is the primary reason why Baghdad International Airport has remained closed to commercial traffic.

A U.S. Army Blackhawk helicopter（黑鷹直升機）crash landed October 25 after taking ground fire near the town of Tikrit north of Baghdad. One crewmember was injured.

On June 12 a U.S. Army Apache helicopter（阿帕奇直升機）was shot down in western Iraq. Its two crewmembers were not injured.