Joe K had shown up as the signature on many letters sent to accommodation addresses. In March 1941 BSC chemists detected secret writing in a Joe K letter; the secret message referred to a duplicate letter sent to "Smith" in China. (The censor working on the Joe K correspondence was Nadya Gardner who conducted chemical tests and found secret writing in some letters, according to historian H. Montgomery Hyde.) The BSC mail-intercept operation was run in coordination with the FBI — even though J. Edgar Hoover, the Director of the FBI, and William Stephenson, the head of the BSC, did not get along. The FBI was able to trace the Smith letter and found that it contained a plan of U.S. defenses in Pearl Harbor.