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. ||||| 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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 reset Latest from White House on suspicious letters PHOTOS: Roger Wicker Play Slideshow 
 
 “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 ||||| Story highlights A Mississippi judge received a similar letter that is also being tested 
 
 Justice Department says Paul Kevin Curtis of Corinth, Mississippi, has been arrested 
 
 Envelopes were addressed to Obama and Sen. Roger Wicker, R-Mississippi 
 
 Initial tests on those envelopes detected the deadly poison ricin; additional tests under way 
 
 An arrest has been made in connection with possibly contaminated letters sent to President Barack Obama and Sen. Roger Wicker, the Department of Justice said Wednesday. 
 
 Paul Kevin Curtis was arrested by the FBI at his home in Corinth, Mississippi, the department said in a statement. 
 
 Discovered Tuesday, the letters were addressed to Wicker, a Mississippi Republican, and to Obama. The justice department release said a third letter was sent to a Mississippi justice official. 
 
 The letters to Wicker and Obama were stopped at a government mail-screening facility after initial tests indicated the presence of the deadly poison ricin. 
 
 Because initial tests can be "inconsistent," the envelopes have been sent off for additional tests, an FBI statement said. The FBI does not expect to receive results from the tests until Thursday, federal law enforcement sources told CNN. 
 
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 The letters read: "To see a wrong and not expose it, is to become a silent partner to its continuance." 
 
 They were signed "I am KC and I approve this message," a source said. 
 
 Reports of suspicious packages and envelopes also came into two Senate office buildings late Wednesday morning. Capitol Police evacuated the first floor of the Hart Senate Office Building for more than an hour and questioned a man in the area who had a backpack containing sealed envelopes, but the man was not taken into custody. 
 
 "It just reminds you that with public service comes the real possibility that you could be a target," said Sen. Pat Roberts, R-Kansas. "But on the other side of it, we have an excellent police force, and I think they'll get to the bottom of it." 
 
 Beyond Washington, suspicious letters spotted 
 
 Sadie Holland, a judge in Lee County, Mississippi, told CNN Wednesday night that she received an envelope with a suspicious substance and a letter similar to the ones sent to the offices of Obama and Wicker. 
 
 Last Wednesday, the judge received and opened a typewritten letter -- postmarked from Memphis, without a return address -- that included "suspicious content," Lee County Sheriff Jim Johnson told reporters. The letter had "great consistencies and similarities" to the letters sent to Obama and Wicker, he said. Investigators were testing the contents of the letter to determine whether ricin was inside, he said. 
 
 Holland told CNN the letter originally tested negative for ricin but was being retested Wednesday. Local authorities were awaiting the test results to determine whether to file state charges, Johnson said. 
 
 "The letter was handled, the chemical was handled by several different individuals in our justice court system," Johnson said, but added that "we do not have any reason to believe that anyone's life is in danger." 
 
 Investigators are trying to determine whether suspicious letters found at Senate offices elsewhere in the country came from the same source, federal law enforcement sources said. 
 
 Sen. Carl Levin, D-Michigan, said one of his home-state offices received a "suspicious-looking" letter and alerted authorities. "We do not know yet if the mail presented a threat," said Levin, the chairman of the Senate Armed Services Committee. 
 
 A staffer for Arizona Sen. Jeff Flake flagged "suspicious letters" at the freshman Republican's Phoenix office, Flake spokeswoman Genevieve Rozansky said in a statement, but "no dangerous material was detected in the letters." 
 
 Phoenix Fire Department spokesman Jonathan Jacobs said the envelope contained some type of powder. The person who initially found the envelope is being treated at a Phoenix-area hospital for a pre-existing condition and stress from the event, and others in the immediate vicinity were examined as well. 
 
 In a statement issued Wednesday, the FBI said it has no indication of a connection between the tainted letters and Monday's bombings at the finish line of the Boston Marathon. But the discoveries further heightened security concerns at a time when Congress is considering politically volatile legislation to toughen gun laws and reform the immigration system. 
 
 White House spokesman Jay Carney said the president had been briefed on the letters. 
 
 "Obviously, he understands and we all understand that there are procedures in place, as the FBI has said. There are -- there is is a process in place that ensures that materials that are suspicious or substances that are found to be suspicious at remote locations are then sent for secondary and more intense testing, and that process is under way now," Carney said. 
 
 A Texas chiropractor's words in the spotlight 
 
 While authorities in Washington investigated the letters, the wife of a Texas chiropractor said the wording in them caught her by surprise. 
 
 The phrase used in the letters is something chiropractor John Raymond Baker once said and has been widely quoted online, his wife, Tammy Bennett Baker, told CNN. 
 
 On Wednesday, she sounded surprised when told by CNN that the wording was included in the letters under scrutiny in Washington. She said she was not aware of the letters and that the phrase refers to her husband's general philosophy of care. She said their office phone started ringing frequently Wednesday afternoon, and it was "kind of freaking out our other employee." 
 
 A 2006 post on a blog for Baker's office says the comment originally was a criticism of insurance companies. Since then, the site says, it "has been a quote that has been picked up and quoted (sometimes without attribution) around the net" and "people are using it about all kinds of injustices." 
 
 The letter sent to Wicker had a Memphis, Tennessee, postmark and no return address, Senate Sergeant-at-Arms Terrance Gainer wrote in an e-mail to senators and aides Tuesday. Wicker has been assigned a protective detail, according to a law enforcement source. 
 
 A laboratory in Maryland confirmed the presence of ricin on the letter addressed to Wicker after initial field tests also indicated the poison was present, according to Gainer. However, the FBI said additional testing was needed because 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. 
 
 A law enforcement source said further tests would be conducted at the Army's biomedical research laboratory at Fort Detrick, Maryland. 
 
 After the arrest was announced Wednesday night, Wicker released a statement thanking "the men and women of the FBI and U.S. Capitol Police for their professionalism and decisive action in keeping our family and staff safe from harm." His offices in Mississippi and Washington "remain open for business to all Mississippians," Wicker said in the statement. 
 
 Mail for members of Congress and the White House has been handled at offsite postal facilities since the 2001 anthrax attacks, which targeted Sen. Patrick Leahy, D-Vermont, and then-Majority Leader Tom Daschle, D-South Dakota. 
 
 Senate mail service shut down 
 
 Senators were told Tuesday that the mail facility would be temporarily shut down "to make sure they get everything squared away," Sen. Claire McCaskill, D-Missouri, said Tuesday afternoon. 
 
 "The bottom line is, the process we have in place worked," she said. 
 
 Senate Minority Leader Mitch McConnell, R-Kentucky, also praised the postal workers and law enforcement officers for "preventing this threat before it even reached the Capitol." 
 
 "They proved that the proactive measures we put in place do, in fact, work," he 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. 
 
 A previous ricin scare hit the Capitol in 2004, when tests identified it in a letter in a Senate mail room 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.
– 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.