Washington (CNN) -- An envelope that tested positive for the deadly poison ricin was intercepted Tuesday afternoon at the U.S. Capitol's off-site mail facility in Washington, congressional and law enforcement sources tell CNN.
Senate Majority Leader Harry Reid said he was told the letter was addressed to the office of Sen. Roger Wicker, R-Mississippi. A laboratory in Maryland confirmed the presence of ricin after an initial field tests indicated the poison was present, Senate Sergeant-at-Arms Terrance Gainer said.
The letter had a Memphis, Tennessee, postmark and no return address, Gainer wrote in an e-mail to senators and aides.
Sen. Claire McCaskill told reporters after a briefing for lawmakers that a suspect has already been identified in the incident, but a knowledgeable source said no one was in custody Tuesday night.
Wicker, the junior senator from Mississippi, has been assigned a protective detail, according to a law enforcement source.