Update 10:18 p.m.: A federal official told The New York Times the letter has been sent to the Federal Bureau of Investigation’s laboratories in Quantico, Va., for further examination because field tests for ricin can be unreliable.

As a precaution, Senate post offices on Capitol Hill were shut down and probably will stay closed for the rest of the week. Senators were told that there was no evidence of contamination inside the Capitol or on the grounds, but investigators were still looking into whether any similar envelopes had been sent to anyone in the House.

Update 8:10 p.m.: According to the Associated Press, authorities already have a suspect in mind for the ricin letter sent to Sen. Roger Wicker. The ricin letter, apparently sent from someone who regularly mails Congress members, lacked a return address but bore a postmark form Memphis, Tenn. There were no “obviously suspicious outside markings” on the ricin letter.

Update 7:20 p.m.: The Associated Press reports that the ricin letter was not delivered to Wicker’s office. Since the 2001 anthrax attacks following 9/11, mail sent to congressional offices has been routinely screened in a separate, off-site facility in Prince George’s County, Md.

"Luckily, this was discovered at the processing center off premises," Sen. Dick Durbin, D-Ill., told the AP, adding that all congressional mail is “roasted, roasted, sliced and opened” before being delivered.

7 p.m.: A poisoned letter sent to Sen. Roger Wicker, R-Miss., has tested positive for ricin, a potentially deadly poison. The poisonus ricin letter was intercepted before it was delivered to Wicker.

Little is known about the sender of the letter or why it was delivered, but according to CNN’s Dana Bash, the letter was tested at a mail facility and confirmed positive for ricin, a deadly poison.

“CNN had learned: early tests on envelope to senator show positive test for ricin poison. Further testing being done now,” Bash wrote on her Twitter feed.

Initially, media outlets were unsure who the intended recipient of the poisoned ricin letter was, but Senate Majority Leader Harry Reid, D-Nev., has confirmed to the Associated Press that the ricin letter was addressed to Republican Roger Wicker, the junior senator from Mississippi.

Politico reports that Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller are currently briefing other senators on safety issues associated with the poisoned ricin letter.

Ricin is an extraordinarily deadly poison derived from the castor oil plant. Typically, the poison is administered through injection or inhalation. According to a European Food Safety Authority report, a minuscule dose (less than 1/200th of an asprin tablet) is enough to kill an able-bodied adult male.

News that Wicker was sent a ricin-poisoned letter comes only one day after the city of Boston was rocked by a pair of explosions set off near the Boston Marathon finish line. The timing between the ricin letter and the Boston bombings bears an uncomfortable similarity to a string of anthrax attacks on government employees and media offices following the Sept. 11, 2001, attacks.

Only one week after the World Trade Center and Pentagon were attacked, two Democratic senators were sent letters containing deadly anthrax spores. Five people, including postal workers, were killed in the anthrax attacks.

The anthrax mailings are the primary reason why congressional mail is now screened in private facilities, a practice which allowed the ricin letter to be caught before it was delivered to Wicker.

This is a breaking news story. Check back for more updates.