In the deadliest day for American troops in Iraq, a U.S. Chinook helicopter was shot down Sunday near Baghdad airport, and at least 15 troops were killed and some 20 were injured, U.S. officials said. Another U.S. soldier was killed early Sunday by a bomb blast in the Iraqi capital. The U.S. Central Command in Tampa, Fla., said the helicopter included soldiers going to the airport for rest and recreation abroad. "The Chinook was shot down by an unknown weapon," a U.S. spokesman in Baghdad said, according to The AP. A short time after the attack, the spokesman in Baghdad said there were 20 wounded, but later, the Army confirmed at least 15 were killed. The Baghdad spokesman said the helicopter was part of a formation of two Chinooks carrying more than 50 passengers to the U.S. base at Baghdad International Airport. Witnesses said they saw two missiles fired at the helicopter. Some Iraqis at the site of the crash were celebrating word of the helicopter downing and also a fresh attack on U.S. soldiers in Fallujah, where witnesses said an explosion struck one vehicle in a U.S. Army convoy at about 9 a.m. Sunday. They claimed four soldiers died, but U.S. military sources said they couldn't confirm the report. Another U.S. soldier was killed early Sunday by a bomb blast in the Iraqi capital, an Army spokesman said. The spokesman said the soldier was evacuated to hospital after the blast, but died a few hours later. **PHOTO CAPTION*** US soldiers inspect the site where a Chinook helicopter was shot down outside the flashpoint town of Fallujah, 50km west of Baghdad. (AFP)