After three arduous war patrols, Morton was given the highly dangerous assignment of penetrating the Sea of Japan for the second time, in October 1943. Morton was reported missing in action that December, when his submarine was presumed lost. After the war, it was determined from Japanese records that, on October 11, in the time frame in which the "Wahoo" was expected to exit through La Pérouse Strait, an antisubmarine aircraft found a surfaced submarine and attacked, dropping three depth charges.