At 19.20 hours on 16 Jan 1944 the Sumner I. Kimball (Master Harry Atkins), a straggler from convoy ON-219 due to bad weather the morning of 16 January, was spotted by U-960 about 900 miles east of Belle Isle Strait in 52°15N/33°45W steering a zigzag course at about 10 knots. The U-boat began chasing the ship in heavy seas and thunder storms and fired at 21.35 hours a stern torpedo which hit after 38 seconds. The ship proceeded at about 7 knots and unsuccessfully tried to ram the U-boat in low visibility shortly after two torpedoes fired in a second attack at 22.02 hours had missed. At 22.39 hours, two more torpedoes were fired of which one hit after 52 seconds and stopped the vessel. The U-boat then dived to reload the torpedo tubes and fired two coups de grâce at 01.36 hours and 02.03 hours, which both hit amidships without visible effects, but three minutes after the U-boat surfaced at 03.15 hours the ship broke in two just aft of the midship house and forward of #4 hatch.

During the night, U-960 searched in vain for survivors and on daylight the wreck of Sumner I. Kimball because the BdU assumed that the target could have been the German blockade runner Rio Grande (which had been scuttled by the crew after being stopped by USS Omaha (CL 4) and USS Jouett (DD 396) on 4 January). On 17 January, the forward section was located at 16.00 hours, photographed and sunk by a coup de grâce at 18.43 hours. No survivors were ever found, eight officers, 32 crewmen and 29 armed guards (the ship was armed with two 3in and eight 20mm guns) were lost.