Michael Mantenuto -- who played Jack O'Callahan in Disney's "Miracle" -- killed himself Monday in Washington State.

The actor, a former University of Maine hockey player, was found dead in his car by police in Des Moines, WA. The coroner tells TMZ ... Michael committed suicide by shooting himself. 

As a budding actor with hockey skills, Michael landed the role in the Disney flick about the 1980 U.S. Olympic hockey team. He only acted in 2 other movies, and went on to enlist in the army ... and joined the Special Forces.

Col. Guillaume Beaurpere, commander of Mantenuto's army unit, announced his death.

He was 35.