I've posted a list on my website of all the documents that were in Rutherford's FBI file, not the documents, but the names of what was in the file when I went through it when I was working on the Proclaimers book. Back in 1990, the file cost over $200.00 to obtain from the government under the Freedom of Information Act.
In the book, “Official and Confidential,” author, Anthony Summers, wrote on page 29 that John Lord O'Brian, head of the War Emergency Division, made J. Edgar Hoover a special agent in his division in 1917. It was O'Brian who was responsible for the investigation and behind the scenes maneuvering to have the Watch Tower Directors arrested and brought to trial. He was known in the Episcopal Church as a "Heretic Hunter." The Sept. 29, 1920 Golden Age has much to say about Mr. O'Brian accusing him of the arrests of Watch Tower officials.
Hoover eventually was made O'Brian's assistant, therefore, knew all about the Watch Tower Espionage trial. Subsequently, his view of Rutherford was reflected in the 1941 Hoover letter posted on my website and Marvin's blog.