Special agent Ryan Bessey was in his office at the New Jersey branch of the U.S. Fish and Wildlife Service, in Galloway, on September 23, 2015, when he took a call from a colleague in the intelligence unit.

If French authorities considered the bust important enough to tell the U.S. about it, Lapkiewicz was worth looking into, Bessey thought. He began to do some digging.

Glass shattered high above Seventh Avenue in Manhattan before dawn on a cold November morning in 1953.

Seconds later, a body hit the sidewalk. Jimmy, the doorman at the Statler hotel, was momentarily stunned. Then he turned and ran into the hotel lobby. “We got a jumper!” he shouted. “We got a jumper!” The night manager peered up through the darkness at his hulking hotel. After a few moments, he picked out a curtain flapping through an open window. It turned out to be room 1018A. Two names were on the registration card...