But Popov couldn't make the FBI take seriously his warning about an attack on Pearl Harbor. As Loftis tells it: The Japanese were pressing the Germans for details about Pearl Harbor defenses, and Jebsen told Popov that the Japanese would be attacking the base as the British had done in a raid at Tarantao, Italy, the prior year. Although Popov warned the FBI in August 1941, Director J. Edgar Hoover didn't tell President Franklin Roosevelt or naval intelligence.