A military investigation has determined that the helicopter pilots that crashed off the Florida coast in March had become disoriented in a thick fog and lost control, failing to switch from visual-based procedures to instrument-based ones.

The March 10 crash during a training exercise killed four Louisiana Army National guardsmen and seven Marines.

A Thursday morning news release from the Louisiana Guard says both pilots aboard the Black Hawk helicopter suffered “spatial disorientation” that caused them to lose control of the aircraft. The statement says the pilots did not effectively transition from visual flight procedures to instrument flight procedures.

The crash was investigated by the Louisiana Guard and the U.S. Special Operations Command.