The U-boat skipper, Hans-Dieter Heinicke, had a crippled submarine and was headed home when he spotted the juicy allied convoy off the coast of North Carolina.

Although Heinicke, 29, had a damaged boat, he had sunk only three ships on his four prior patrols and probably saw the 19 merchant vessels of convoy KS-520 plodding south at 8 knots as a chance to redeem himself.

About 4 p.m. on July 15, 1942, about 30 miles off Cape Hatteras, U-576 attacked. In the ensuing free-for-all, the sub sank one ship and damaged two others but was assailed by aircraft and escorts, and sank with all hands.

On Tuesday, researchers announced that they had discovered the wreck of U-576, as well as the wreck of the sunken merchant ship, and hailed the find as a rare snapshot of a little known chapter of World War II.

The two ships were found in August in 690 feet of water a few hundred yards apart following a five-year search headed by the National Oceanic and Atmospheric Administration (NOAA).

The German U-576 departs Saint-Nazaire, France, on the Atlantic coast, in the early 1940s. The submarine was sunk in 1942 by aircraft fire. (Ed Caram/Courtesy photo, via NOAA)

In making the announcement, NOAA also released a striking series of wartime photographs of U-576 and its crew that had been gathered by a North Carolina
U-boat historian, Ed Caram, who died last year.

NOAA also made public a sonar image taken of the U-boat on the bottom of the ocean, where it is resting on its starboard side.

“This, to my mind, is just a monument to World War II history,” said NOAA maritime archaeologist Joe Hoyt, the chief investigator on the project. “We have this notion generally that World War II happened in Europe and the South Pacific.”

Here is “a World War II battlefield that’s literally right in our back yard,” he said. “The ocean kind of swallows up these stories. And now we have the ability and the technology to go out and kind of get them back.”

The battle off Cape Hatteras came during a period of intense warfare between convoys and
U-boats in the area in the first six months of U.S. involvement in World War II, said Hoyt, who is based in Newport News, Va.

Allied ships were headed up and down the East Coast, and
U-boats preyed on them, Hoyt said.

An estimated 90 vessels — including four U-boats — were sunk off North Carolina between January and July 1942. It was “almost a ship every other day going down,” he said.

By July 1942, though, the convoys were heavily guarded by aircraft and warships. Convoy KS-520 was escorted by five Navy and Coast Guard vessels as well as Navy aircraft.

U-576, with its heraldic lion motif painted on the conning tower, had suffered serious and irreparable damage to its main ballast tank from an earlier aircraft attack, Hoyt said. And the skipper had decided to head for home.

But the sub hadn’t bagged a single ship since it had left its base in St. Nazaire, France, on June 16, and Heinicke probably was tempted by the convoy, Hoyt said.

“They got quite a lot of reward when they returned to Germany, based on the level of tonnage that they sunk,” Hoyt said. If you sank a lot of ships, “you get a parade in Berlin, and all the girls love you.”

“So to return from a patrol, which would often be more than a month, without having really sunk much would be a pretty big bummer for these guys,” he said.

Yet even before the U-boat could attack, it was detected by a Coast Guard escort, which dropped depth charges.

The submerged boat closed in anyway and fired four torpedoes, sinking the old Nicaraguan flagged freighter Bluefields and damaging two other ships without causing loss of life, a NOAA account of the battle said.

Hoyt said that because of the lost weight of the four torpedoes and the broken ballast system, the U-boat couldn’t stay submerged and popped to the surface in the middle of the convoy. “They were immediately pounced on,” he said.

An armed merchant vessel opened fire with its deck gun, and two Navy escort planes swooped in and dropped more depth charges. The U-boat sank with its crew, most of whom were probably in their 20s, Hoyt said.

Several of Caram’s photographs show members of the crew in happier times posing or horsing around on deck.

Hoyt said researchers knew the location of three of the U-boats sunk off North Carolina but were not sure of U-576’s location. “Nobody knew where it was,” he said.

In partnership with East Carolina University and the federal Bureau of Ocean Energy Management, NOAA in 2009 began searching the waters where the convoy battle was known to have happened.

During a series of expeditions, researchers using data from archives and small robot submersibles, scoured the ocean floor with sonar until they located the likely wreck of the Bluefields last year.

Knowing the U-boat must be nearby, they focused their search for the sub earlier this year around the presumed Bluefields wreck. “Low and behold, another target . . . was picked up,” Hoyt said. But the image was vague, and a sharper one was needed.

In August, the scientists went back for a closer look. Hoyt said the team was huddled around a computer screen on a NOAA research ship as sonar data was downloaded from a submersible.

“We’re kind of sweating it out,” he said.

The first image that came through was from an odd angle and was not clear, but after a small adjustment, the unmistakable sonar profile of a submarine appeared.

“Everyone was just like, ‘Whoaaa!’ ” he said. “It was pretty great.

“We were all hugging each and going nuts,” he said. “There was much celebration. . . . It was that clear. It was totally unmistakable.”

NOAA said the German government still owns the U-boat and has asked the United States to protect the site. The U.S. recognizes German ownership and has pledged its protection, NOAA said.

U-576 is now “a war grave,” Hoyt said. “We believe there are 45 German sailors inside.”