Sen. Mary Landrieu. D-La., said Capitol Police intercepted a letter at an off-site screening center Tuesday and later advised senators none of their offices were in danger, The Hill reported.
Government officials said they haven't linked the letter with Monday's terrorist attack on the Boston Marathon in which two bombs exploded at the finish line, killing three people and injured more than 170.
"There was a letter sent to a member that had gone through our processing facility -- not on-site here, but in the area -- and it was identified as containing ricin, which is a dangerous substance," Landrieu said after senators were briefed on Boston Marathon investigation.
Police said they didn't think anyone in the Capitol complex was in danger, Landrieu said.
Senate Majority Leader Harry Reid, D-Nev., said he was unaware of any other dangerous letters.
Sen. Claire McCaskill, D-Mo., said police have a suspect, who has been known to write letters to congressional members, The Hill said.
The off-site mail facility will be closed for the next few days so the FBI and Capitol Police can investigate and conduct more tests, Senate Sergeant at Arms Terrance W. Gainer said in a statement.
Gainer said the letter had a Memphis postmark but no return address.
Wicker's office referred all calls to police.
Ricin, which occurs naturally in castor beans, is highly toxic when inhaled, ingested or injected.
In 2001, several letters containing anthrax were mailed to senators in the wake of the Sept. 11 terrorist attacks. After the 2001 anthrax attacks, all mail sent to the U.S. Capitol went through testing at a screening facility in Capitol Heights, Md.