Late on 29 February, "Bronstein" made radar contact with an unknown vessel. "Bostwick" and Thomas joined her in "boxing" the target, which was revealed by flares to be a surfaced German submarine, . "Bronstein" opened fire with her guns, but the submarine managed to submerge. The three destroyer escorts tracked the U-boat, raining down a barrage of depth charges on her estimated positions. At 0324, "Thomas" dropped a pattern of charges that produced a huge underwater explosion, the last sounds heard from "U-709".