An envelope mailed to Sen. Roger Wicker’s Capitol office tested positive for the poison Ricin.
CNN reported that the envelope arrived at the Senate mail facility and was tested. When it tested positive for Ricin, it was retested twice.
On Tuesday evening, senators were notified of the incident during a briefing with Secretary of Homeland Security Janet Napolitano and FBI Director Robert Mueller. The briefing was scheduled to discuss cybersecurity legislation.
Congressional post offices will be shut down, temporarily, senators were told.
Sen. Claire McCaskill told Politico that the envelope was sent by someone who writes to lawmakers often.
Ricin is a poison derived from castor beans. It is highly toxic, and a very small dose can be fatal.