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Letter with ricin sent to Mississippi senator

The United States Capitol on Saturday, Dec. 29, 2012.

April 16, 2013 12:00:00 AM PDT
A letter addressed to Mississippi Sen. Roger Wicker tested positive for ricin in a field test at the Senate mail facility in Landover, Maryland, according to federal law enforcement sources.

A source told ABC News that the letter is being tested in a lab for a definitive result. Senate Majority Leader Harry Reid said Capitol offices are on high alert.

All mail delivery to Capitol Hill has been stopped as a precaution.

Homeland Security Secretary Janet Napolitano declined to comment on the suspicious letter.