Historians have resolved a 70-year-old mystery surrounding USS Indianapolis and the number of World War II troops who survived when the Navy cruiser was torpedoed by the Japanese on July 30, 1945.

The sinking of the ship, which went down in under an hour, has been called the U.S. Navy’s worst-ever disaster at sea. Out about 1,200 crewmen aboard, it was long believed that 880 sailors and Marines lost their lives during the attack itself or in the water following the ship’s sinking, due to extreme conditions including shark attacks, dehydration and saltwater poisoning.

However, thanks to new research by the Naval History and Heritage Command Communication and Outreach Division, officials have since confirmed that 879 people died following the attack on USS Indianapolis. In addition, the number of people who survived is 316, as the Navy has long said, and not 317 as some have posited over the years.

The problem was always a matter of math. Now, the reason the number of survivors was revised downward by one is that the total number of people on board was revised too, from 1,196 to 1,195. A search of available public records found that Clarence William Donnor (pictured below), a Navy radio technician believed to have been among the Indianapolis crew, had continued to serve until he was discharged in 1946. But he wasn’t a 317th survivor. In fact, he had not been on the ship in the first place.