The sender listed the FBI's old Dallas address on Lamar as the return address on some of the letters.
The person who mailed dozens of letters from Dallas containing a white powder to governors' offices and American embassies listed current or former FBI offices as a return address, the FBI said Friday.
Investigators released new details and photographs of the typewritten mailings in the hope of prompting new leads.
The letters, sent to more than 40 governors' offices nationwide and to U.S. embassies overseas, were postmarked from the Dallas area on Dec. 4, 8 and 11, the FBI said.
Tests showed the white powder was harmless, but mailing threatening letters is still illegal, FBI spokesman Mark White said.
The FBI said each envelope has contained a similar letter. Investigators did not release the contents of the letters, but said its message is "not clear."
Some of the letters were postmarked "North Texas," which indicates they were processed in Coppell, the FBI said.
Some letters listed a return address of 1801 North Lamar in Dallas. The FBI's Dallas field office was located on Lamar until moving to its new headquarters on Justice Way several years ago.
The sender also wrote a name with the return address, but the FBI did not disclose that information.
The U.S. Postal Inspection Service, which is investigating the case with the FBI, is offering a $100,000 reward for information leading to the conviction of the sender.