At 23.22 hours on 13 Mar, 1943, the Keystone (Master John Henry Darby) was torpedoed by U-172 about 450 miles west of the Azores. The ship had been in convoy UGS-6, but straggled on 12 March about 15 miles behind the convoy due to engine problems and a destroyer remained with the freighter all night, but left the next morning to rejoin the convoy. The U-boat reported the vessel under her former name Sage Brush.
One torpedo struck on the port side aft of the #5 hatch. The explosion blew a hole in the hull between #5 hold and the poop deck, destroyed the steering engine and steering gear, buckled the deck, disabled the 4in stern gun, flooded the shaft alley and killed one armed guard and a fireman on watch below. Five minutes after the hit a fire started and one of the aircraft carried on deck caught fire. The engines were secured and most of the eight officers, 35 crewmen, 27 armed guards (the ship was armed with one 4in and nine 20mm guns) and two passengers (US Army) abandoned ship in good order in four lifeboats. After the crew cleared the vessel, a coup de grâce was fired at 23.55 hours, which struck at the #3 hold and broke the ship in two. Both parts sank until 00.27 hours on 14 March. The survivors were picked up after seven hours by the Portuguese steam merchant Sines and landed at Horta in the Azores on 16 March.