SPOKANE, Wash. -

Matthew Buquet, already charged with sending threatening correspondence to the local post office, allegedly mailed a letter with ricin to President Barack Obama.

This according to the FBI, which said Thursday that several threatening letters posted to Fairchild Air Force Base, the CIA and Obama were allegedly sent by Buquet.

Buquet, 38, is already in custody on a charge of mailing threatening correspondence after two letters, which tested positive, were found at the downtown post office addressed both to the Spokane Post Office and Federal Judge Fred Van Sickle.

Thursday the FBI confirmed that on May 22 a grand jury indicted him for mailing threatening communications for the letters sent to the downtown post office and Van Sickle.

The letter sent to Obama was intercepted May 22, a letter sent to Fairchild Air Force Base was also discovered on May 22 and another letter sent to the CIA in McLean, Va., was detected May 22 but was sent to a location that does not receive mail deliveries and as of Thursday has not been located. The four letters in law enforcement hands right now were postmarked in Spokane on May 13.

The FBI confirmed that the letter sent to Obama did test positive for ricin, a poison derived from castor beans that has no known antidote.

The contents of the letter sent to Fairchild Air Force Base are still undergoing analysis by the FBI.

FBI officials added that the letters sent allegedly by Buquet are similar to the ones that were postmarked in Shreveport, Louisiana and sent to NYC Mayor Michael Bloomberg. The Shreveport letters are separate from the Spokane ricin investigation.

Buquet, who is in federal custody, is only facing one charge of mailing threatening communications for the letters sent to the downtown post office and Van Sickle. He has not been indicted on the three additional letters sent to Obama, the CIA or Fairchild Air Force Base.