( 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 .
The FBI says four similar letters were postmarked the same day , with similar ink . Tests show three of those letters had ricin ; tests are pending on the fourth one . The FBI is looking for a letter that was directed to the CIA . All five letters are believed to have been sent from Spokane around May 13 .