An anonymous reader writes "A letter addressed to Senator Roger Wicker (R-Mississippi) was tested and found to contain ricin, a highly toxic, inexpensive, and easily produced substance derived from castor beans. The letter was intercepted at the U.S. Capitol's off-site mail facility and nobody has been injured. The letter was postmarked Memphis, Tennessee, but listed no return address. Sen. Claire McCaskill told reporters that a suspect has been identified." And, this morning, a letter addressed to the President was discovered containing a suspicious substance. Update: 04/17 16:25 GMT by U L : And the substance is ricin. Apparently, air filters at another facility have also tested positive for ricin.