A hundred years ago, a mysterious explosion hit the only major U.S. warship to sink during World War I. Now the Navy believes it has the answer to what doomed the USS San Diego: An underwater mine set by a German submarine cruising in waters just miles from New York City.

That’s the conclusion of an investigation by scientists, archaeologists and historians convened by the U.S. Naval History and Heritage Command. Last summer, the researchers sent an unmanned underwater vessel to inspect the site off New York’s Long Island. Their analysis ruled out a torpedo and sabotage, two other possible scenarios.

The San Diego was sailing to New York on July 19, 1918, when an external explosion near the engine room shook the armored cruiser. Water rushed into the hull. Within minutes, the 500-foot warship began to capsize. Weighed down with 2,900 tons of coal for a planned voyage across the Atlantic Ocean, the vessel sank in less than a half hour. Six crew members died.