A letter addressed to a U.S. senator from Mississippi tested positive for the dangerous poison ricin, the FBI said Tuesday.
U.S. Capitol Police, which is partnering with the FBI in this investigation, said an envelope containing a white granular substance was intercepted about 11 a.m. Tuesday.
The FBI Field Office in Baltimore said the first test in the field was preliminary. The letter must now be sent for further testing.
The letter was addressed to Sen. Roger Wicker, R-Miss. It was intercepted at the Landover, Md., mail facility and did not reach the Capitol.
Wicker's office has informed the senator's close associates of the tainted letters, a source close to Wicker tells NBC News. The office also is telling associates that no one at the post office was exposed to the substance.
"This matter is part of an ongoing investigation by the United States Capitol Police and FBI," said a statement issued by Wicker. "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."
The letter's appearance "wouldn't raise suspicion," the Senate sergeant at arms said. It was postmarked Memphis, Tenn.
Mail to Senate offices is suspended likely until the end of the week, according to Sens. Jeff Flake, R-Ariz., and Angus King, I-Maine. House leadership said there has been no change in their mail service.
"It was caught at the screening facility,” said Sen. Claire McCaskill, D-Mo. “That's why we have the offsite screening facility for mail. And the tests came back positive. And they are shutting down the post offices temporarily to make sure they get everything squared away and we are notifying our state offices what to look for."
McCaskill suggested that officials identified a person of interest.
"Evidently this person, the person that is a suspect, writes to a lot of members," she said.
The off-site screening facility used to review all mail sent to Congress is temporarily shut down while a search is under way for the possibility of any other tainted letters. They are reviewing mail postmarked Memphis.
Ricin is far less dangerous than the anthrax that was found in letters sent in 2001, officials said.