Federal officials said a letter addressed to a U.S. senator was discovered to contain a potential poison. It was intercepted at an off-site facility in Landover where congressional mail has been examined before delivery since anthrax-laced letters were sent to Capitol Hill in 2001.

The letter was addressed to Sen. Roger Wicker (R-Miss.) and initially tested positive for ricin, but officials familiar with the case said it was undergoing further testing late Tuesday. The officials gave no indication why the letter was sent to the second-term senator.

In a statement, Wicker thanked federal authorities “for their hard work and diligence in keeping those of us who work in the Capitol complex safe.”

During an afternoon briefing on the attacks at Monday’s Boston Marathon, senators were told that there is no suspected link between the letter and the attacks.

Senate Sergeant at Arms Terrance Gainer, who oversees Senate security, said that the suspicious letter was discovered Tuesday and that authorities conducted multiple field tests, followed by laboratory confirmation tests.

“I have confidence in our procedures, our personnel, the United States Capitol Police response personnel, the strength and weaknesses of field testing and the need for laboratory confirmation,” Gainer said in an e-mail.

But FBI spokesman Paul Bresson said late Tuesday that “initial field tests produced mixed results, so the substance is in the process of undergoing further analysis at an accredited laboratory. Only after that test result can a determination be made about whether the substance is ricin.”

A U.S. Capitol Police statement said that preliminary tests indicated the substance found was ricin but that the letter was being forwarded “to an accredited laboratory for further analysis.”

Gainer had briefed senators on the letter and outlined steps they should take to reassure staffers how to handle suspicious mail. “While we have no indication that there are other suspect mailings, it is imperative to follow all mail handling protocols,” Gainer wrote in a letter sent to all Senate staff Tuesday evening.

The off-site facility that screens mail will be closed for several days for further testing and examination by federal investigators, delaying delivery of mail to the Capitol and Senate office buildings.

Previous incidents have led to negative tests of suspicious letters. A false positive happened in 2004, when a letter to then-Senate Majority Leader Bill Frist (R-Tenn.) was believed to contain ricin.

Ricin, a toxin found in castor beans, is poisonous if inhaled, injected or ingested, according to the National Counterterrorism Center. Treatment is available, but long-term organ damage is the likely result of exposure.

Eleven years ago, anthrax spores delivered in letters killed five people, injured 17 and changed how the U.S. Postal Service thinks about mail safety.

In his letter to staff members, Gainer gave no insight into any suspects. He said the letter, postmarked from Memphis, contained no return address and no “outwardly suspicious” markings.

Sen. Claire McCaskill (D-Mo.), however, told reporters that the investigators are focusing on a person who has regularly written to members of Congress in the past.

The U.S. Postal Service spends about $100 million annually to screen every piece of first-class mail sent or received by U.S. households and mail sent to federal addresses in Washington, say officials familiar with the process.

Mail destined for the White House, Congress and federal agencies requires closer scrutiny. In the past, postal officials have said that once mail with federal addresses is sorted at a Washington facility, it is trucked it to a New Jersey irradiation facility operated by a private firm operating medical sterilization equipment. The Postal Service spends about $12 million annually on irradiation, say officials familiar with the process.

Mark Brady, a Prince George’s County Fire/EMS Department spokesman, said federal authorities notified local crews Tuesday morning that they were investigating a “potential package” at the facility in Landover. Several hours later, federal authorities requested crews come assist in that investigation by having an ambulance on standby to possibly treat those working on the package.

Such incidents, Brady said, are typical — occurring once or twice a day.

Sari Horwitz and Matt Zapotosky contributed to this report.

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