Law enforcement officials say other letters sent to the U.S. Senate and even President Obama are being tested for ricin.
The FBI says preliminary tests on a letter sent to the president indicate the presence of poisonous ricin.
The letter is undergoing further testing because preliminary field tests can be unreliable, creating false positives.
The letter was intercepted at a facility away from the White House. It comes the day after officials said a letter sent to Sen. Roger Wicker tested positive for poisonous ricin. That letter to Wicker, a Republican, was intercepted at a Senate mail facility just outside Washington.
The FBI says the letters to Obama and Wicker are related and are both postmarked out of Memphis, Tenn., dated April 8.
In an intelligence bulletin obtained by The Associated Press, the FBI says the letters both say: "To see a wrong and not expose it, is to become a silent partner to its continuance." Both letters are signed, "I am KC and I approve this message."
The FBI says it is pursuing investigative leads to determine who sent the letters.
The FBI says there is no indication of a connection to the bombing at Monday's Boston Marathon.
Letters that were considered suspicious were also addressed to Alabama Sen. Richard Shelby but it was later cleared. Michigan Sen. Carl Levin. Levin said his regional office in Saginaw received a suspicious letter but his aide did not open it.
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