The U.S. military in Iraq says hostile fire may have been responsible for the crash of two U.S. Army Black Hawk helicopters in the northern city of Mosul on Saturday. Seventeen soldiers were killed in the crash, which was initially thought to be an accident. The two Black Hawk helicopters, traveling after sunset, crashed in a residential neighborhood just west of downtown Mosul. Eyewitnesses say the aircraft collided in mid-air before going down.

They also report that one of the Black Hawks may have been hit by a rocket-propelled grenade before it crashed. The U.S. military, which is still investigating the crash, says that report is speculative at best.

But privately, some officials acknowledge that one of the helicopters may have come under hostile ground fire and swerved upward to avoid being hit. The move could have driven the aircraft's rotor blades into the second helicopter, causing both to crash.

Both helicopters belonged to the Army's 101st Airborne Division, which is in charge of security in Mosul.

If the Black Hawks crashed as a result of hostile fire, it would be the fourth and fifth helicopters to be brought down by insurgents in the past three weeks.

On November 2, insurgents near Fallujah, west of Baghdad, shot down a U.S. Army Chinook helicopter, killing 16 soldiers.

Five days later, insurgents shot down a Black Hawk helicopter near Saddam Hussein's hometown of Tikrit, killing all six soldiers aboard. Late last month, another Black Hawk helicopter was hit by a rocket-propelled grenade. No one was killed, but five soldiers were wounded.

Saturday's crashes pushed the number of Americans killed in combat this month to more than 60.

Mosul, Iraq's third largest city, has been relatively peaceful until recently. For the past several weeks, U.S. troops have come under numerous attacks from what the military believes is a well-armed and well-financed mix of Saddam loyalists, Islamic militants and foreign fighters.