An envelope sent to the Washington, D.C., office of Sen. Roger Wicker (R-Miss.) included a substance that has tested positive for the toxic substance ricin. 
 
 The Senate mail facility on Tuesday notified the U.S. Capitol Police that it had received an envelope containing “a white granular substance,” according to a release. The envelope “was immediately quarantined” at the off-site facility, and Capitol Police hazardous material personnel responded. 
 
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 “Preliminary tests indicate the substance found was Ricin,” said the police statement, which was released late Tuesday night. “The material is being forwarded to an accredited laboratory for further analysis.” 
 
 (PHOTOS: Roger Wicker's career) 
 
 The Capitol Police says it is partnering with the FBI for an “ongoing investigation.” The statement added that the Capitol complex has not been affected. 
 
 The U.S. Senate mail facility is currently closed for more testing — all mail bound for Capitol Hill is now being directed through a U.S. House facility. 
 
 The letter was postmarked Memphis, Tenn., and had no suspicious markings or return address, according to a message sent to congressional staffers by Senate Sergeant at Arms Terrance Gainer. Gainer indicated that the Senate mail facility would be closed for two to three days “while testing and the law enforcement investigation continues.” 
 
 (Also on POLITICO: Napolitano: No sign of 'broader plot') 
 
 Sen. Claire McCaskill (D-Mo.) said members were briefed that the substance had been found in a letter and that a suspect has been identified. 
 
 McCaskill said the letter came from an individual who frequently writes to lawmakers. She wouldn’t identify the person but confirmed officials were investigating someone. 
 
 McCaskill said state offices have been told what to look for if there are more letters containing the toxic substance. 
 
 (Also on POLITICO: What is ricin?) 
 
 The Senate went into recess shortly after 6 p.m. Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller began briefing senators shortly afterward about the incident. A Homeland Security official said that the briefing was intended to address cybersecurity but that the subject pivoted after the ricin substance was discovered. 
 
 All congressional mail is screened ever since Capitol Hill-bound letters laced with anthrax were discovered in October 2001, which contributed to the death of five people and infected 17 more. Senate offices were closed for several days in 2004 when ricin was found in the offices of then-Senate Majority Leader Bill Frist (R-Tenn.). 
 
 Follow @politico ||||| Federal officials discovered Tuesday a poison-laced letter sent to Sen. Roger Wicker (R-Miss.), uncovering the material at an off-site location where congressional mail has been screened since anthrax-laced letters were sent to Capitol Hill in 2001. 
 
 Senate Majority Leader Harry M. Reid (D-Nev.) and other senators exiting an FBI briefing on the Boston attacks, told reporters that the letter was tested at the facility and came up positive for ricin. Officials gave no indication why the letter was sent to Wicker, a low-profile senator in his second term. ||||| Story highlights Laboratory tests confirmed the presence of ricin, Senate official says 
 
 The letter had a Memphis, Tennessee, postmark and no return address 
 
 It was addressed to Sen. Roger Wicker, R-Mississippi 
 
 Ricin is a highly toxic substance derived from castor beans 
 
 An envelope that tested positive for the deadly poison ricin was intercepted Tuesday afternoon at the U.S. Capitol's off-site mail facility in Washington, congressional and law enforcement sources tell CNN. 
 
 Senate Majority Leader Harry Reid said he was told the letter was addressed to the office of Sen. Roger Wicker, R-Mississippi. A laboratory in Maryland confirmed the presence of ricin after initial field tests indicated the poison was present, according to Senate Sergeant-at-Arms Terrance Gainer. 
 
 However, the FBI said additional testing is needed as field and preliminary tests produce inconsistent results. 
 
 "Only a full analysis performed at an accredited laboratory can determine the presence of a biological agent such as ricin," according to the bureau. "Those tests are in the process of being conducted and generally take from 24 to 48 hours." 
 
 A law enforcement source said more tests would be conducted at the Army's biomedical research laboratory at Fort Detrick, Maryland. 
 
 The letter had a Memphis, Tennessee, postmark and no return address, Gainer wrote in an e-mail to senators and aides. 
 
 Sen. Claire McCaskill told reporters after a briefing for lawmakers that a suspect has already been identified in the incident, but a knowledgeable source said no one was in custody Tuesday night. 
 
 Wicker, the junior senator from Mississippi, has been assigned a protective detail, according to a law enforcement source. 
 
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 Postal workers started handling mail at a site off Capitol Hill after the 2001 anthrax attacks that targeted then-Majority Leader Tom Daschle and Vermont Sen. Patrick Leahy, among others. Senators were told the post office would be temporarily shut down "to make sure they get everything squared away," McCaskill, D-Missouri, said Tuesday afternoon. 
 
 "The bottom line is, the process we have in place worked," she said. Members will be warning their home-state offices to look out for similar letters, she added. 
 
 A previous ricin scare hit the Capitol in 2004, when tests identified a letter in a Senate mailroom that served then-Majority Leader Bill Frist's office. The discovery forced 16 employees to go through decontamination procedures, but no one reported any ill effects afterward, Frist said. 
 
 Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms -- an amount the size of the head of a pin -- can kill an adult. There is no specific test for exposure and no antidote once exposed. 
 
 It can be produced easily and cheaply, and authorities in several countries have investigated links between suspect extremists and ricin. But experts say it is more effective on individuals than as a weapon of mass destruction. 
 
 Ricin was used in the 1978 assassination of Bulgarian dissident Georgi Markov. The author, who had defected nine years earlier, was jabbed by the tip of an umbrella while waiting for a bus in London and died four days later. 
 
 Wicker, 61, was first appointed by former Republican Gov. Haley Barbour to the U.S. Senate in December 2007 after the resignation of then-Sen. Trent Lott. He was then elected to the seat in 2008 and won re-election in 2012 to a second term. 
 
 Before joining the Senate, he was a U.S. representative in the House from 1995 to 2007. Before that, he served in the Mississippi Senate.
– Scary story breaking out of DC: A letter addressed to Republican Mississippi Sen. Roger Wicker tested positive for the poison ricin, reports Politico. The envelope never got to him, reports CNN, which says it was intercepted at an off-site facility that sorts mail for the Capitol. Three separate tests turned up positive for the highly toxic poison. Authorities have identified a suspect, said Sen. Claire McCaskill, adding only that it was someone who writes lots of letters to lawmakers. It's not clear why the apparent target was Wicker, described by the Washington Post as a "low-profile senator in his second term." (He replaced Trent Lott in 2008.) The envelope got detected at a facility set up in 2001 after some senators were sent letters tainted with anthrax. "The bottom line is, the process we have in place worked," said McCaskill. FBI chief Robert Mueller and Homeland Security chief Janet Napolitano briefed senators today, though it's not clear when the letter was received.