A sailor aboard the USS Nebraska, based at Naval Base Kitsap-Bangor, was found dead Monday aboard the submarine.

Machinist Mate Fireman William Mack, 21, of South Pittsburg, Tenn. was on his first deployment aboard the submarine, which left port March 18 on a strategic deterrence patrol, said Lt. Kellie Randall, spokeswoman for Submarine Group Nine.

Mack’s death is under investigation.