An FBI officials confirms to FOX13 News that a suspicious letter was sent to U.S. Sen. Roger Wicker (R-MS) and it was intercepted at a postal facility in Maryland.
FOX News said the tainted letter sent to the senator from Mississippi contained ricin.
FOX News Congressional reporter Chad Pergram said the letter had a Memphis, Tenn., postmark with no return address.
One senator, Claire McCaskill of Missouri, said authorities have a suspect in the fast-moving ricin case, but she did not say if an arrest had been made. She added the letter was from an individual who frequently writes lawmakers.
Ricin is a highly toxic poison found naturally in castor beans, according to the Centers for Disease Control. Ricin can be in the form of a powder, a mist, or a pellet, or it can be dissolved in water or weak acid.
All mail sent to Congressional Offices in Washington, D.C., goes through the Congressional Mail Sorting Office screening facility in Maryland. It's designed to intercept these kind of threats. The letter was discovered at a mail processing plant in Prince George's County in suburban Maryland, said Sen. Dick Durbin, D-Ill.
Senate Majority Leader Harry Reid confirmed to the Associated Press a letter containing ricin or another poison was sent to the office of Sen. Wicker.
In brief remarks to reporters in the Capitol, Reid did not say when the letter was sent.
Wicker's office issued a statement saying "any inquiries regarding member security must be directed to the United States Capitol Police."
Capitol Police had no immediate comment.
The FBI and U.S. Capitol Police are both investigating.
"This matter is part of an ongoing investigation by the United States Capitol Police and FBI," Sen. Wicker said in a statement Tuesday evening. "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. Gayle and I appreciate everyone's thoughts and prayers."
Sen. Mary Landrieu also confirmed a letter containing poison had been to a sent to a senator. She said lawmakers were informed of the fact at a closed-door briefing about the Boston Marathon bombings.
One law enforcement official said evidence of ricin appeared on preliminary field tests of the letter, although such results are not deemed conclusive without further testing. The official spoke on condition of anonymity because the investigation remains active.
Terrance W. Gainer, the Senate sergeant-at-arms, said in an emailed message to Senate offices that the envelope to Wicker had no obviously suspicious outside markings.
He added there was "no indication that there are other suspect mailings." Yet he urged caution, and also said the Senate off-site mail facility where the initial tests were performed on the letter will be closed for a few days while the investigation continues.
The discovery evoked memories of the days after the terrorist attacks of Sept. 11, 2001, when mail laced with anthrax began appearing in post offices, newsrooms and congressional offices.
That included letters sent to Sen. Tom Daschle, D-S.D., who was Senate majority leader, and Sen. Patrick Leahy, D-Vt. Two Senate office buildings were closed during that investigation.
Overall, five people died and 17 others became ill. The FBI attributed the attack to a government scientist who committed suicide in 2008.
More immediately, though, the discovery came as lawmakers were demanding answers to the attacks in Boston a day earlier.
There was no evidence of a connection between the bombings and the letter addressed to Wicker, a Mississippi Republican.
Associated Press reporter Donna Cassata in Washington, D.C., and writers Andrew Miga, Seth Borenstein, Eric Tucker and Eileen Sullivan contributed to this report.