Authorities believe a letter sent to President Obama containing a suspicious substance that tested positive for ricin came from the same person who mailed an envelope that was also filled with the deadly toxin to a Republican senator. In both instances, the letters did not reach the intended target.

The discovery of the letter to the president comes one day after it was revealed that a piece of mail addressed to Sen. Roger Wicker of Mississippi had tested positive for ricin. A federal official told NBC News that the two letters were “very similar.”

Meantime, ABC News’ Terry Moran reports that authorities have intercepted a second letter sent to the U.S. Senate containing a suspicious substance that also tested positive for the poison. It’s not clear yet who the intended recipient was.

NBC News:

Federal officials told NBC News that they believe they know who sent the letters, but no arrest was made. Authorities were waiting for further test results.

Ricin is made from castor beans and can kill within 36 hours. There is no antidote. Some threatening letters simply contain ground castor beans, resulting in a positive field test for ricin without the concentrated poison. Results from full laboratory tests are expected in the next 24 to 48 hours.

An FBI official told NBC News that the agency did not initially believe the letters were related to the attack on the Boston Marathon on Monday.

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As Gawker noted though, “there’s a history of ricin false positives that have been used to justify hasty security action.”

— Posted by Tracy Bloom.