(CNN) -- The FBI announced Thursday its agents are looking for a letter, this one directed to the Central Intelligence Agency, that it believes was similar to at least three other letters that contained ricin.
Letters were sent to a federal judge, a Spokane post office, President Barack Obama and Fairchild Air Force Base, the FBI said. Tests are being conducted on the letter sent to Fairchild Air Force Base to determine whether it -- like those other three letters -- contained ricin.
The letter sent to the CIA was "addressed in a manner consistent with the first four envelopes" and went to "a location that does not receive mail deliveries."
As of Thursday evening, authorities have not been able to locate this letter. It was not immediately clear how they knew about its existence.
Eight days ago, 37-year-old Matthew Ryan Buquet was arrested as part of the investigation into the aforementioned letter sent to U.S. District Court Judge Fred Van Sickle. That letter contained a message threatening to kill and injure the judge.
Both that letter and one sent to a Spokane post office were intercepted on May 14, while the letters to Obama and Fairchild Air Force Base were tracked down on May 22.
The four envelopes that authorities have in their possession were all postmarked out of Spokane on May 13, the FBI said. Each one of them had addresses handwritten in red ink.
The bureau said it believes the fifth letter -- written to the Central Intelligence Agency at its headquarters in McLean, Virginia -- was likely postmarked the same day and also had red ink on its envelope.
The FBI's announcement Thursday out of Washington state came the same day officials intercepted a letter addressed to Obama.
The off-site facility that screens mail addressed to the White House turned the letter over to the FBI Joint Terrorism Task Force for testing and investigation, U.S. Secret Service spokesman Brian Leary said.
This letter was similar to threatening letters sent to New York Mayor Bloomberg and the gun-control group he founded. Preliminary tests indicate ricin was in those latter letters, New York Deputy Police Commissioner Paul Browne said Wednesday.
If inhaled, injected or ingested, less than a pinpoint of ricin can kill a person within 36 to 48 hours because of the failure of the respiratory and circulatory systems. There is no known antidote for the toxin, which is derived from castor beans.