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 . The FBI says three similar letters were postmarked May 13 , 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 FBI . All five letters are believed to have been sent from U.S. around the same day .
The FBI says three similar letters were postmarked May 13 , 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 FBI . All five letters are believed to have been sent from U.S. around the same day .