The Secret Service announced Wednesday that a letter addressed to President Obama containing a "suspicious substance" was intercepted at the White House's off-site mail facility. The FBI says a preliminary test indicates poisonous ricin, which is derived from castor beans, was in the letter sent to Obama, the Associated Press reports.

The letter arrived on Tuesday, the same day as another letter sent to Sen. Roger Wicker (R-Mississippi) believed to also contain ricin, which was also intercepted. Since the 2001 anthrax scares, all mail sent to Congress is screened at an off-site facility. It is believed that the same person sent both letters.

"Monday's attack in Boston reminded us that terrorism can still strike anywhere at any time," Senate Minority Leader Mitch McConnell said Wednesday. "And as yesterday's news of an attempt to send ricin to the Capitol reminds us, it is as important as ever to take the steps necessary to protect Americans from those who would do us harm."

Parts of two Senate buildings were temporarily closed as U.S. Capitol police investigated a "suspicious" package received at the office of Sen. Richard Shelby (R-Alabama) and suspicious envelopes being delivered to Senate offices. In addition, Michigan Sen. Carl Levin says a suspicious package was sent to his district office in Saginaw.
"The investigation into these letters remains ongoing, and more letters may still be received. There is no indication of a connection to the attack in Boston," the FBI said in a statement.

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(Photo: REUTERS/Larry Downing)