One of the most successful hunter killer anti-submarine groups was Task Group 21.11 which was formed around USS Block Island CVE-21. The escort carrier air crews and the destroyers in the group sank six U-boats before May 29, 1944, the seventh was sunk on that day. The skill and courage of the men in TG-21.11 was not the full reason for their success, they were helped by Bletchley Park and ULTRA.

The last U-boat sunk by TG-21.11 departed Lorient, France on May 14, 1944 and within hours of her departure the cryptologists at Bletchley Park had learned of her departure, TG-21.11 was informed and Block Island and her destroyers moved to intercept. It was thought by the Americans that the boat was heading to Argentina on a special mission to deliver spies, however there seems to be no evidence that confirms that this was indeed the mission.

On May 28 at 0100 hrs. the boat was located by Lt. (j.g.) Carl Mansell in an Avenger, but shortly after contact his radar failed and he was not able to attack. The boat, U-549 and her commander, Kapitänleutnant Detlev Krankenhagen submerged and escaped into the darkness. Captain Francis M. Hughes, commanding officer of Block Island, had no intention of giving up easy, aircraft searched throughout the night and day, one getting a radar contact, but the boat remained unseen.

The next day, May 29, 1944, more search planes were sent aloft, but the boat, which was trying to get into a firing position, was not located. At 1700 hrs. Block Island launched six FM-2 Wildcats and began to stage six TBM Avengers and another Wildcat, but reports of bad weather caused the captain to rethink this and he reduced the number of Avengers to two.

Meanwhile Krankenhagen the 26 year old commander of U-549, rather than trying to escape, made a daring move through the screen of escorting destroyers and lined up on the carrier. He had been on only one previous war patrol, but did not see any combat with the exception of being attacked from the air at least once. At about 2000 hrs. he fired two torpedoes (some sources claim three) hitting the ship near the port bow and near the stern seconds later.

The ship lost power immediately and began to sink, one of the escorting destroyers, USS Barr DE-576, saw the periscope and dropped a pattern of 13 depth charges, but all missed. The depth charge attack was answered with a torpedo in the stern of Barr exploding her remaining depth charges and killing a number of her crew.
U-549 twenty minutes later slammed a third torpedo into Block Island which finished the carrier. As the situation on Block Island got worse her commander ordered the ship abandoned and two destroyers, USS Ahrens DE-575 and USS Robert I. Paine DE-578 came alongside to take off survivors. The six Wildcats in the air could not return to Block Island and were directed to the Canary Islands, but due to lack of fuel four crashed into the sea, the pilots were not recovered.

The two small ships took on about 951 survivors and Block Island, the only U.S. carrier lost in the Battle of the Atlantic, slid below the surface. As she went deeper explosions were felt on the surface ships, presumably her depth charges going off, which almost caused a panic on the destroyers as the men thought they had been hit by a torpedo.

The situation on Barr was stabilized by her crew and her commander, Lt. Commander Henry H. Love had his wounded and most of the crew transferred to USS Eugene E. Elmore DE-686 keeping only an emergency crew onboard. Her stern was destroyed and seventeen men had been killed, but she did not sink, Elmore took her in tow and was later relieved by USS Wilhoite DE-397 and the Dutch tug Antic, she ultimately was towed into Casablanca, where all the survivors had been landed.

During the rescue operation the submarine was picked up on sonar by USS Ahrens DE-575, her commander, Lt. Commander Morgan H, Harris, alerted Lt. Commander George L. Conkey on Elmore who pursued the target and fired Hedgehog mortars at the submerged boat. After hearing many explosions deep under the water it was clear that U-549 had been sunk, of the fifty-seven men in the boat none survived. U-boat command declared the boat missing as of Aug. 4, 1944 with no known reason for her loss.

The navy announced the loss of Block Island on June 5, but did not release the cause, Barr was repaired and saw further action in the Pacific, she was sunk as a target in 1963.