On 14 June 1944, "Thresher" headed out for her 13th war patrol, again commanded by MacMillan. 25 June, she joined a "wolfpack" that also included , , and . Nicknamed the "Mickey Finns" and under the overall command of Captain William V. "Mickey" O'Regan (flying his flag in "Guardfish"), the group picked up "ditching signals" from a downed aircraft that afternoon and changed course to investigate. Arriving in the vicinity on 27 June, they found only a drop tank and no trace of plane or pilot.