A suspicious letter addressed to Republican Sen. Roger Wicker of Mississippi twice tested positive for the poison Ricin.
One law enforcement official said evidence of ricin appeared on two preliminary field tests of the letter, although such results are not deemed conclusive without further testing. The official spoke to the Associated Press on condition of anonymity because the investigation remains active.
In a statement issued Tuesday, Wicker thanked law enforcement officials for their response.
“This matter is part of an ongoing investigation by the United States Capitol Police and FBI. I want to thank our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe," Wicker said. "Gayle and I appreciate everyone’s thoughts and prayers.”
Sen. Claire McCaskill of Missouri said authorities have a suspect in the fast-moving ricin case, but she did not say if an arrest had been made. She added the letter was from an individual who frequently writes lawmakers.
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The FBI and U.S. Capitol Police are both investigating. Both declined to comment.
McCaskill told CNN that the suspect letter never made it to the Capitol. "It was caught in the screening facility. That's why we have an off-site screening facility for mail," she said, according to the Raycom News Network.
The Obama administration has expressed concern in the past that a dangerous regional arm of al-Qaida was trying to produce the deadly poison ricin to use in attacks against the United States.
Citing unnamed intelligence officials and classified intelligence reports, The New York Times has also reported in the past that an al-Qaida affiliate in Yemen was trying to acquire large quantities of castor beans, used to produce ricin.
The publication said the apparent intent was to pack the poison around small explosives that could be exploded to disperse the ricin, a white powdery substance so deadly that a speck can kill if inhaled or taken into the bloodstream.
Wicker, 61, was appointed to the Senate in 2007 and won election to a full term two years ago. He previously served a dozen years in the House.
He has a solidly conservative voting record, so much so that he drew notice last week when he voted to allow debate to begin on controversial gun legislation in the Senate. "I cast this vote at the request of the National Rifle Association, of which I am a member," he said in a statement at the time that added he has a 100 percent voting record in favor of Second Amendment rights.
In 2004 ricin was found in the Dirksen Senate Office Building office of then Senate Majority Leader Bill Frist. The building and two others were closed for a week as crews tested some 1,300 air and surface samples, but “only a few” taken from Frist’s office tested positive for ricin.
Terrance W. Gainer, the Senate sergeant-at-arms, said in an emailed message to Senate offices that the envelope to Wicker had no obviously suspicious outside markings and lacked a return address. It bore a postmark from Memphis, Tenn.
The U.S. Senate's chief security office said on Tuesday that the Senate's mail facility will be closed for the next two to three days pending further investigation.
The FBI, the Capitol Police and other agencies are involved in the investigation, the Senate Sergeant at Arms said in an email to Senate employees.
On Monday, Wicker tweeted his condolences to those caught up in the attack on the Boston Marathon.
"I am saddened to learn the tragic news in Boston. My thoughts and prayers are with the runners, families, and first responders," the senator tweeted.
Reuters and the Associated Press contributed to this report.
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