president obama*As each day passes since the Boston Bombing, it becomes apparent that somebody is sending a message. It’s not exactly clear what specifically that message is, but it’s definitely not good.

The latest example are letters containing poisonous ricin sent to Mississippi Sen. Roger Wicker and President Obama. The question is who’s doing this and why? It is believed that the letters came from the same person.

The letter to Obama was intercepted at an off-site White House mail facility Tuesday and was being tested further, a Secret Service official told NBC News. 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.

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.

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.

Read/learn more at NBC News.

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