U.S. officials said they had intercepted a letter sent to Senator Roger Wicker that tested positive on Tuesday for the deadly poison ricin, and that the U.S. Capitol police, FBI and other agencies had launched an investigation.
The letter was postmarked from Memphis, Tennessee, and had no return address, Terrance Gaines, the Senate sergeant at arms, said in a warning to members of the Senate.
"Senate employees should be vigilant in their mail handling processes for ALL mailings," Gaines said in a written statement.
Members of the Senate were briefed on the ricin incident by Gaines during a meeting with FBI Director Robert Mueller and Janet Napolitano, the secretary of Homeland Security, on Tuesday on the bombings in Boston.
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Several senators told reporters after the briefing that incident reminded them of the anthrax attacks in the wake of the Sept. 11, 2001, attacks on the Pentagon and World Trade Center.
The positive ricin test came one day after the explosions at the Boston Marathon that killed three people and injured 176.
"I don't know if it's a coincidence. It's too early to tell. We don't know enough about Boston," said Senator Richard Durbin, the No. 2 Democrat in the Senate.
Senate Majority Leader Harry Reid said he had been told the letter was addressed to Wicker, a Republican senator from Mississippi.
All mail to the U.S. Senate had been stopped, and post offices at the Capitol had been closed as a precaution, the senators said. They were getting in touch with their state offices, where mail is not subject to the same extensive screening, to ensure that precautions were being put in place.
Many senators expressed concern about their staffs and the risks to postal workers.
They said they were aware of only one letter that had been intercepted that tested positive for ricin.
It was not immediately clear whether Wicker had attended the briefing by Mueller and Napolitano that was open to all senators from both parties.
Wicker issued a statement saying only that the matter was being investigated and expressing gratitude for thoughts and prayers on his behalf.
"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," he said in the statement.