BAGHDAD, Iraq – Four U.S. Marines died when a Sea Knight helicopter plunged into a lake in volatile Anbar province, the military said Monday, raising the number of American troops killed during a bloody weekend in Iraq to 13.

It was the second military aircraft to go down in a week in Anbar, a stronghold of Sunni insurgents, although the military said mechanical problems rather than gunfire had forced the emergency landing on Sunday.

“The pilots maintained control of the aircraft the entire time,” the military said.

A Marine was pulled from the water but attempts to resuscitate him were unsuccessful. The bodies of three missing Marines were found in a subsequent search, the military said. Twelve other passengers survived.

High U.S. casualties and doubts about the war were considered a major factor in the U.S. Democratic congressional victory.