At 08.03 hours on 15 June 1944, U-621 fired one Gnat torpedo at convoy EPL-8 and hit USS LST-133 (Lt Floyd E. Richards, USN) which was about 2000 yards behind station, steaming at full speed of 10 knots with two Rhino tugs in tow about 27 miles northeast of Barfleur, France. The U-boat observed how the vessel broke in two and then managed to retreat without being attacked by the escorts as it was assumed that the vessel had struck an acoustic mine. However, the landing ship remained intact from frame 41 forward so the Germans probably mistook the Rhino tugs that drifted away after the hit as parts of the vessel. The explosion blew away the greater part of the fantail and both 40mm Bofors gun tubes on the stern, demolished the crew quarters and steering engine room and left the vessel without propulsion as the screws and rudder were destroyed. The deck house was damaged by the stern anchor winch that was blown forward and large pieces of twisted deck plate hurled through the air fell on deck and the vehicles stored there. The landing ship carried the men and equipment of the HQ and three batteries of the 113th Field Artillery Battalion, 30th US Infantry Division. As breakfast had just been served many men were washing their mess kits on the fantail when the torpedo struck and the casualties were high: 15 crew members and 28 passengers (22 US Army and 6 USN Seabees) were killed and 17 crew members and 11 passengers (8 US Army and 3 USN Seabees) were wounded.

USS Raven (AM 55) and USS Chickadee (AM 59) en route to Portsmouth went alongside to take off the wounded, the former taking USS LST-133 in tow towards Omaha Beach until relieved by USS Arikara (ATF 98) at 12.35 hours. Eleven hours after the hit, the landing ship was beached with the help of USS LCI(L)-490 and USS LCI(L)-84 and unloaded until 02.00 hours on 16 June. Afterwards the ship was sufficient repaired to permit towing her to Southampton by the British tug Empire Meadow on 18 June. USS LST-133 was prepared at Barry for a transatlantic crossing and returned to service after being repaired in the US.