WASHINGTON, D.C. (CBS NEWS) -- A letter sent to Sen. Roger Wicker, R-Miss., may have contained ricin, according to a preliminary field test taken by the FBI and U.S. Capitol Police.
The letter, which was intercepted by the Capitol Police at a Prince George's County, Md., processing center and never reached Wicker's office, has been sent to the Baltimore FBI office for further testing. A source tells CBS News that it's common for false positives to come back on field tests of ricin.
There's no additional information on who sent the letter or what message may have accompanied the letter.
CBS News has learned that results from the more definitive test to be taken at the Baltimore FBI office will not be available tonight.
At a senators-only briefing about the Boston Marathon bombing this afternoon by FBI Director Robert Mueller and Homeland Security Janet Napolitano, Sen. Mary Landrieu, D-La., said senators were also briefed by Sergeant at Arms Terence Gainer about the Wicker letter.
Landrieu told reporters that the senators were told the suspected letter writer writes a lot of letters to members and added she believes authorities have someone in custody.
As a precaution, the Senate post office has been temporarily shut down.