A "white granular substance" in an envelope addressed to Sen. Roger Wicker (R-Miss.) has tested positive for the poison ricin, authorities said late Tuesday.
US Capitol Police were notified that an envelope containing the suspicious substance had arrived at an off-site mail facility and was immediately quarantined.
The envelope, which had a Tennessee postmark and no return address, never reached the senator's desk.
"Preliminary tests indicate the substance found was ricin," said a statement from Capitol Police, which was released late Tuesday night. "The material is being forwarded to an accredited laboratory for further analysis.”
Ricin is a toxic substance found naturally in castor beans that can be fatal when inhaled.
Inhaling ricin can cause difficulty breathing, fever, cough, nausea, and tightness in the chest leading to potentially fatal respiratory failure, according to the Centers for Disease Control and Prevention.
"We were told it's not as deadly as anthrax, but it's still very serious," Senate Majority Whip Dick Durbin, (D-Ill.) told USA Today after a briefing on the poison attempt.
Durbin expressed confidence that the mail-screening system put in place after the 2001 anthrax attacks was working.
"Everything that's sent to us, everything that's mailed to us, is roasted, toasted, sliced and opened," he said.
Senate Sergeant at Arms Terrance Gainer said that the Senate mail facility would likely be closed for two to three days "while testing and the law enforcement investigation continues."