(Image Credit: Lance Cpl. Christopher O'Quinn/U.S. Marine Corps)

A "likely unavoidable" bird strike was responsible for a helicopter crash that killed two Camp Pendleton Marines in September, according to a Marine Corps investigation.

Capt. Jeffrey Bland, 37, and 1st Lt. Thomas Heitmann, 27, were killed on Sept. 19 when their AH-1W Cobra helicopter collided with a female red-tailed hawk near Fallbrook, Calif.

The bird, which investigators estimated had a 4-foot wingspan and weighed 3 pounds,  hit the top of the helicopter and damaged the pitch change link, according to a report obtained by the San Diego Union-Tribune through a public records request.

Vibrations in the main rotor caused the helicopter to fall to the ground in three separate pieces, the report stated.

The two Marines were killed on impact, while the wreckage ignited a brush fire that spread over 120 acres.