In July 1918, the 15,000-ton armored cruiser USS San Diego sank off Long Island, New York, losing six sailors from a crew of 1,200 after a mysterious explosion struck the vessel.

The ship was returning home after escorting U.S. troops and cargo ships across the perilous North Atlantic passage to Europe, defending convoys against marauding German U-boats and transforming the course of the conflict by delivering 10,000 doughboys a day to the Allied Powers.

Now it had been felled just eight miles from New York Harbor.

San Diego remains the only major U.S. warship sunk in World War I. Until now, the cause of the explosion was a mystery. Some experts thought that a German saboteur had smuggled a bomb on board. Others were convinced that a torpedo fired by a German U-boat was to blame, even though lookouts never saw the tell-tale bubble trail left on the water’s surface.

But now military historians and scientists have finally confirmed an initial Navy court of inquiry finding that a German-laid underwater mine sank the warship.

A team of U.S. military and oceanographic experts presented the results of a two-year investigation on December 11, 2018, at the annual meeting of the American Geophysical Union in Washington, D.C., bringing to a close a century of debate.

“We believe the U-156 [U-boat] sunk the USS San Diego and used a mine to do so,” said Alexis Catsambis, a maritime archaeologist at the U.S. Naval History and Heritage Command.

The scientific data gleaned from the investigation backs up reports at the time that a U-boat had been patrolling the area. In fact, the U-156 had been spotted by other ships and was one of Germany’s most successful, sinking a total of 44 Allied ships in only 13 months of sea patrols.

After a refueling stop in Portsmouth, N.H., the 1,180 crewmembers of the San Diego were a few hours away from a night on the town. Some had already changed into their spiffy white liberty uniforms as the ship approached the entrance to New York Harbor. As the armored cruiser passed eight miles south of Fire Island, a powerful explosion hit just below the midpoint of the ship.

Captain Harley Hannibal Cristy gave the order to abandon ship after only 15 minutes and it only took less than a half hour for the 500-foot ship to capsize on its side and sink in about 100 feet of water. Six men died; the rest were rescued by lifeboats and passing ships.

The team of investigators, which included experts from 10 federal agencies and academic institutions, used a new kind of predictive computer program developed by the Navy to simulate the flooding of the ship. They combined the computer program with another prediction model for the effects of an explosion on the metal hull, according to Ken Nahshon, a research engineer at the Naval Surface Warfare Center Carderock Division.

Nahshon said the crew of the USS San Diego did everything correctly in responding to the explosion. The problem was more likely in the design of the ship’s interior coal storage compartments, which flooded quickly and were unable to contain the seawater.

“The eye-opening part to us was that there was so many openings inside the ship,” Nahshon said.

Catsambis and his team said the mine explosion did little damage to the ship, but that seawater breached a series of bulkheads and caused the top-heavy ship to topple over quickly. Crews had packed the San Diego with coal for the return trip to Europe, including 150 tons sitting on the open deck.

The researchers made six trips to the USS San Diego wreck site from 2016 to June 2018. They used sidescan sonar and underwater robots with laser imaging technology to construct a three-dimensional image of the wreck. Teams of Navy divers added additional information about the general condition of the wreck. Over the years, the wreck has kept intact by the ship’s iron “torpedo belt” installed along the waterline. It’s also become a popular site for both sport divers and fishermen.

Navy officials decided to launch the investigation to solve the mystery once and for all, and to memorialize the six sailors who perished on the centennial of their deaths. “We now know more than ever before about what likely happened during the USS San Diego’s final moments,” said Catsambis.

In a case of cosmic karma, the same German submarine that placed that mine, the U-156, was itself sank by American mines in the eastern Atlantic. It also launched the only known attack on American soil when it fired on tugboats docked in Orleans, Massachusetts.