A helicopter crash in Iraq in January that killed 30 Marines and a sailor — the deadliest air tragedy in more than two years of combat — resulted from human error, not mechanical failure or hostile fire, according to a Marine investigation.

The crew of the CH-53E Super Stallion (search) became disoriented when the weather turned bad, and flew the helicopter into the ground, the investigative report said.

The helicopter was taking troops to western Iraq to guard polling places during the Iraqi elections when it went down in the desert on the night of Jan. 26. A second helicopter made the trip safely.

The pilot of the second craft, whose responsibilities included providing updated weather information to both crews, failed to provide data to the doomed helicopter, according to the report. He was removed from flying status.

The flight crew was from Miramar Marine Corps Air Station (search) in San Diego. The troops were from bases in Hawaii.