German U-Boat POWs were interrogated at a secret facility simply known as P.O. Box 1142. In actuality, the location was Ft. Hunt outside of Washington, DC in Northern Virginia. The over one hundred buildings erected during the war are gone now, the land now a national park run by the US Park Service.
In 2006, a Washington Post reporter wrote two very fine stories on the history of Ft. Hunt, the efforts of a young Park Serivce Ranger, Brandon Bies, to gather the history, and a re-union organized by Bies and the Park Service of the very frail men, who, as robust youngsters, once interrogated U-Boat POWs there. When discharged, these men were told never to speak of what they had done and they kept their silence into the 21st Century.
This is sad. Why? Unknown to the men who had interrogated German U-Boat crews, documents from Ft. Hunt about those interrogations were declassified in the mid-1970s.
Ft. Hunt was first mentioned by historian John Hammond Moore in his fascinating book, The Faustball Tunnel: German POWs in America and Their Great Escape. I learned about Ft. Hunt when I interviewed Dr. Moore in 1980.
[Image of Ft. Hunt courtesy of Wikipedia.]