“The letter to Obama was intercepted at an off-site White House mail facility and was being tested further, the FBI said. A federal law enforcement official said that the letter was “very similar” to one addressed to Sen. Roger Wicker, R-Miss.
Federal officials told NBC News that they believe they know who sent the letters, but no arrest was made because authorities were waiting for further test results.
Filters at a second government mail screening facility also tested positive for ricin in preliminary screening Wednesday.
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.