An envelope sent to an office of Sen. Roger Wicker (R-Miss.) tested positive for the deadly poison ricin, according to a report in Politico.

CNN reported that the envelope was intercepted at the U.S. Capitol's off-site mail facility in Washington, D.C., on Tuesday. It was tested three times for ricin and tested positive all three times.

A Wicker spokesperson said that the investigation was "ongoing" and directed inquiries to Capitol police. Capitol police didn't immediately respond to repeated inquiries.

NBC's Kasie Hunt reported that mail to the Senate has been halted, likely through the end of the week. According to CNN, Sen. Claire McCaskill (D-Mo.) told reporters that police have already identified a suspect.

According to the Centers for Disease Control and Prevention, ricin is a poison naturally found in castor beans. It gets inside the cells of a person's body, preventing the cells from making any proteins. It can come in the forms of a powder, a mist, or a pellet, or it can be dissolved in water or weak acid.

A ricin scare in 2004 temporarily shut down three Senate offices. Postal workers began sorting through an off-site after the 2001 anthrax attacks that targeted, among others, then-Majority Leader Tom Daschle and Vermont Sen. Patrick Leahy.