WASHINGTON -- An envelope laced with the lethal poison Ricin and addressed to a U.S. senator was found at a Maryland mail processing facility, officials said Tuesday.
The envelope, intended for the office of Sen. Roger Wicker (R-Miss.), was discovered at a location that screens letters bound for congressional offices in Washington, according to a law enforcement official. More tests are being conducted to confirm the substance.
Senate Majority Leader Harry Reid (D-Nev.) said only one congressional office was apparently targeted.
Wicker’s officer referred inquiries to the U.S. Capitol Police.
Senators were told of the situation late Tuesday during a classified briefing to discuss security matters, primarily the Boston Marathon bombing, with Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller. The Senate sergeant at arms was also present.
Post offices on the House and Senate side were closed down, and they will go through regular decontamination procedures as a precaution, according to other senators at the briefing.
Aides to Maryland's two Democratic senators, Barbara A. Mikulski and Ben Cardin, said they were not aware of any suspicious letters sent to their offices.
Sue Walitsky, a Cardin spokesman, noted that mail to all congressional offices, including theirs, is being held for inspection and also that staff had been asked to set aside unopened mail in the office. Walitsky said she doesn't yet know how long the hold will last.
Ricin is a poison that can be deadly in small amounts, according to the Center for Disease Control. It can be inhaled or ingested, and there is no antidote.
Mailings to the Capitol complex undergo intensive security screenings that were enhanced after letters containing anthrax were sent to congressional offices following the Sept. 11, 2001, terrorist attacks.
The suspicious envelope tested positive for Ricin after an initial screening, according to a law enforcement official. It is unclear when the envelope was first intercepted.
The FBI's Baltimore field office is working with the Capitol Police to investigate the source of the envelope.
“The bottom line is the process we have in place worked,” Sen. Claire McCaskill (D-Mo.) said as she emerged from the briefing.
She suggested that the sender was known to authorities. “Evidently this person, this person, who is a suspect, writes to a lot of members,” she said.
Terrance W. Gainer, the Senate sergeant at arms, said in a letter to Senate staffers that the envelope was not "outwardly suspicious."
He said it had no return address and was postmarked from Memphis, Tenn.
“While we have no indication that there are other suspect mailings, it is imperative to follow all mail handling protocols,” Gainer's letter said.
For lawmakers’ state offices, Gainer suggested senators use the “Postal Sentry” system – a desktop device that provides airflow and filtration to reduce the threat from harmful agents while opening mail.
The off-site facility is expected to be closed for two or three days as the investigation continues, officials said.
“We will have to notify our state offices, which don't have the screening facilities to look out for it,” said Sen. Angus King (I-Maine).
"Obviously it's concerning,” said Sen. Jeff Flake (R-Ariz.) “It rarely gets to a member before it goes through a lot of staff. That's a big concern, obviously, for all of us. We're very anxious to get more details."
Baltimore Sun reporter Matthew Hay Brown contributed to this article.