In April 1944, a German submarine prowling the waters off Nantucket torpedoed an American tanker caught straggling behind its convoy. The U-boat took cover beneath the sinking ship to avoid detection, but the flagship USS Joyce closed in and delivered a punishing depth charge attack that forced the damaged vessel to the surface.

Under fierce attack, U-550 sank stern first. There it lay, its final resting place an enduring mystery for nearly 70 years.

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But earlier this week, after years of research and days of painstaking searches of the ocean floor, a crew discovered the elusive craft about 70 miles south of Nantucket. Crew members said the submarine was among the last undiscovered German warships along the eastern seaboard, where it once attacked merchant ships and forced blackouts in coastal cities.

“They’ve looked for it for over 20 years,” said Joe Mazraani, a shipwreck diver from New Jersey. “It’s another World War II mystery solved.”

The seven-person crew announced the finding Friday, and said they would be returning to document the wreck site.

Over two voyages, the seven-man crew searched 100 square miles of ocean, using sonar to survey the ocean depths. Traveling at five knots, the ship scanned the vast expanse for signs of the sunken vessel, a tedious process crew members likened to “mowing the lawn.”

Just when it looked as if the search would end in vain, the downed ship came into view. A second scan confirmed the finding, setting off a wild celebration at sea.

“It was jubilation,” Mazraani said. “We were jumping up and down, giving each other high-fives. It was awesome.”

Mazraani dove down to confirm the discovery with pictures. Sonar images provided by the crew show the 250-foot submarine sitting upright and tilted to the starboard side. The images and the general location match that of the sunken German ship.

“She’s totally intact, amazingly,” said Garry Kozak, a specialist in undersea searches who works for a Wareham firm that designs sonar imaging systems.

With only a general idea where the submarine went down that April day, the crew had to sweep a vast area, making the eventual discovery that much more satisfying.

“The gods were with us,” Kosak said. “It’s fantastic when it pops up on the screen and you know you’ve solved the mystery.”

The crew would not divulge the exact location of the wreck, or the depths involved, so that others would not disturb the site. But they said the wreck can be reached by expert divers.

Embarking from the tip of Long Island, the crew traveled for half a day before beginning their search. They had studied naval records in search of clues to the submarine’s whereabouts, but could only narrow the location so much. That, coupled with unpredictable weather conditions, posed a steep challenge.

“The area is very large because of the uncertainty,” Kosak said. “And clearly just getting there is an ordeal.”

Because the submarine had eluded discovery for so long, crew members were beginning to doubt she could be found. As the hours passed, hope dwindled.

“It was really nerve-wracking,” Mazraani said. “We were down to our last hour.”

The group began searching the area last summer after several years of planning and research.

After the submarine was forced to surface, the Germans began firing their deck guns, according to a US Coast Guard history. Three US ships returned fire, and one rammed into it. There were 44 casualties, and 12 survivors.

The prisoners of war, and the survivors of the American tanker, were taken to Great Britain.

The crew plans to notify families of those involved in the battle, including the German casualties.

“It’s their final resting place,” Mazraani said. “There’s a certain reverence to it.”