A U.S. Capitol Police hazmat vehicle is parked at a mail processing facility for Congressional mail in Prince George's County where a letter addressed to Sen. Roger Wicker, R-Miss., tested positive for ricin, Tuesday, April 16, 2013, in Hyattsville, Md. An envelope addressed Wicker tested positive Tuesday for ricin, a potentially fatal poison, congressional officials said, heightening concerns about terrorism a day after a bombing killed three and left more than 170 injured at the Boston Marathon. (AP Photo/Molly Riley)
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