Kohler and Chatterton traveled to Washington, DC, where they used information from the schematics to narrow their search to 20 submarines. They compared the Navy’s records of submarine encounters off the East Coast of the US during World War II with official assessments made afterward that predicted the probable outcome of each encounter. Using this method, they narrowed their search to U-857 and U-879. They found that in 1945, both ships were docked in Norway, along with U-869, Horenburg’s boat. That might explain the knife: Horenburg may have lent it to a friend, or someone may have taken it while onshore.

They were shocked to find that the records of sailors in the US Navy shortly after the incidents often contradicted the official accounts written afterward. They found that on April 5, 1945, the destroyer USS Gustafson detected an underwater object near Boston and fired on it. Crewmen heard an explosion and smelled oil, but no one saw any wreckage or oil float to the surface. The initial report from the US Navy stated that if U-857 was sunk, it was likely not a result of this attack. However, the later official report made by postwar assessors stated that the submarine was “probably sunk.