An envelope addressed to a U.S. Senate office tested positive for the deadly poison ricin Tuesday, launching a criminal investigation and prompting warnings to other offices to take precautions with their mail.
The letter, which news reports said was addressed to Sen. Roger F. Wicker, a two-term Mississippi Republican, was intercepted at the Capitol's off-site mail facility Tuesday.
"While we have no indication that there are other suspect mailings, it is imperative to follow all mail handling protocols," Terrance W. Gainer, the Senate sergeant at arms, said in a letter to all Senate employees late Tuesday evening.
According to The Associated Press, Sen. Claire McCaskill, Missouri Democrat, said the letter was from an individual who frequently writes lawmakers and that there is a suspect in the case.
Mr. Gainer said the Capitol Police, the FBI and other law enforcement agencies are investigating the mailing, and he said the off-site Senate mail facility will be shut down for several days while testing is conducted.
Mr. Gainer said the letter was postmarked from Memphis, Tenn., but had no "outwardly suspicious" markings.
Mr. Wicker's office issued a statement directing inquiries to the U.S. Capitol Police.
Senate Majority Leader Harry Reid, Nevada Democrat, told reporters of the test, and other lawmakers said they had been briefed by the office of the Senate sergeant at arms.
Sen. Mark Begich, Alaska Democrat, said that although he is always concerned about the safety of his office, "obviously the Senate security here is very strong."
CNN reported that the letter was tested three times and each test came up positive for poison.
According to the Centers for Disease Control and Prevention, unintentional exposure to the poison, naturally found in castor beans, is unlikely unless someone eats the beans.
In 1978, Georgi Markov, an anti-communist Bulgarian writer and journalist who was living in London, died after a man pricked him with an umbrella that had been rigged to inject a poison ricin pellet under Markov's skin. In the 1940s, the U.S. military experimented with using ricin as a germ-warfare agent, according to the CDC.
The latest incident was reminiscent of the weeks after the Sept. 11, 2001, terrorist attacks on the U.S., when letters with the poison anthrax were mailed to some Senate offices, as well as several offices throughout the country.
More than two dozen people at the Capitol tested positive for exposure to anthrax from the letters, and thousands of people who were working at the Capitol at the time were given antibiotics as a precautionary measure.
Across the country, five people died from exposure to anthrax, including two postal workers in the Washington area who may have handled the letters addressed to Senate offices.
Nobody was ever put on trial for those mailings, though prosecutors identified Bruce E. Ivins, a scientist who had worked at a biodefense lab at Fort Detrick in Frederick, Md., as the culprit. As prosecutors were preparing to charge him, Ivins committed suicide by taking an overdose of acetaminophen.
In the wake of that incident, all mail to the Capitol is now received and tested at the off-site facility.
In his letter to Senate employees Tuesday, Mr. Gainor warned those who work in offices in the states to use Postal Sentry, a device that is supposed to decontaminate mail.
"The Postal Sentry is really the only defense available in state offices for this type of threat," Mr. Gainor said.
⦁ Sean Lengell and Seth McLaughlin contributed to this article, which is based in part on wire service reports.
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