WASHINGTON — A mystery that went unsolved for 73 years began when Herman Mulligan threw a grenade.

In the thick of some of the most vicious fighting of World War II, on the island of Okinawa, Private First Class Mulligan’s grenade clattered into the dark maw of a Japanese bunker and blew up a cache of ammunition. The huge explosion obliterated most of the hillside, and blasted the 21-year-old Marine beyond recognition.

Amid the chaos, his unidentified body was buried in a hasty battlefield grave, while the Marine Corps listed Private Mulligan as missing in action. In the years after the war, he was reclassified as “unrecoverable,” and the family that knew him gradually died off, until his memory was almost as lost as his bones.

The private’s story could have ended there, among the roughly 72,000 American troops from World War II who have not been accounted for. But the ending has been rewritten by a black-and-white snapshot found in a Marine veteran’s trunk.

The photo inspired an informal network of volunteer sleuths to track down survivors of the battle, pore over forgotten maps and comb through yellowed files until they had traced Private Mulligan’s likely remains to a burial plot under a marble cross in Manila. Then they found a cousin, James Patterson, who could provide DNA for matching.