US authorities intercepted a letter sent to Mississippi Senator Roger Wicker yesterday that preliminary tests showed contained the deadly poison ricin.
The letter has been sent for further analysis to an accredited laboratory, Capitol police said last night.
It was postmarked from Memphis, Tennessee, and had no return address, Terrance Gainer, the Senate sergeant at arms, said earlier in a warning to members of the Senate.
Mr Gainer first said in a statement that the substance had tested positive for ricin, which is found naturally in castor beans and can cause death from exposure to as little as a pinhead amount, usually within the first 72 hours.
Police later issued a statement saying the test was “preliminary” and “indicated” that ricin had been found in the letter.
It was intercepted in a mail handling facility and quarantined, the statement said.
“Senate employees should be vigilant in their mail handling processes for ALL mailings,” Mr Gainer said in his written statement.
Members of the Senate were briefed on the incident by Mr Gainer during a meeting with FBI Director Robert Mueller and Janet Napolitano, the secretary of Homeland Security, yesterday on the bombings in Boston.
Several senators told reporters after the briefing that the incident reminded them of the anthrax attacks in the wake of the September 11 2001, attacks on the Pentagon and World Trade Center.
The 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.
All mail to the US 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.
Mr 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.
Mr Wicker, a former member of the House of Representatives, has been a member of the Senate since he was first appointed to a vacant seat in December 2007. He won a special election to serve the remainder of that term, and was re-elected in November 2012 to a full six-year term.