A letter sent from Memphis, Tenn. and addressed to President Barack Obama has tested positive for ricin and is believed to be from the same sender who mailed a ricin letter to Sen. Roger Wicker.
An off-site White House mail facility intercepted the letter and it is now being tested. Officials believe they know who sent them, as the person often sends letters to Congress, but they are waiting for further test results.
Sen. Claire McCaskill, D-Mo., said, "The person that is a suspect writes a lot of letters to members."
If inhaled, ricin leads to fever, trouble breathing and sometimes death. If it is touched, it can lead to a rash that is not usually fatal. It's made from castor beans.
On Wednesday, filters at a second government mail screening facility tested positive for ricin.
The FBI said it does not believe that the letters are related to the Boston Marathon attack.
Wicker's letter was intercepted on Tuesday at a postal facility in Maryland. During a briefing, other senators were made aware of the letter.
Sometimes, preliminary tests produce inconsistent results, so the exact nature of the substance will not be certain until a laboratory performs a full analysis. The FBI said that some threatening letters have contained ground up castor beans instead of pure ricin.
An FBI bulletin said the letter sent to Obama and Wicker included the phrase: "To see a wrong and not expose it, is to become a silent partner to its continuance."
The letters were signed, "I am KC and I approve this message."