U.S. congressional officials said a letter sent to Republican Senator Roger Wicker of the southern state of Mississippi has tested positive for the deadly poison ricin.
The letter was found at a facility that handles mail going to the U.S. Capitol. Officials said it tested positive for ricin in three separate tests.
It is not likely the letter ever reached Senator Wicker's office.
Ricin is highly poisonous. Just a tiny drop can kill an adult. It is found naturally in castor beans.
Tuesday's discovery evoked memories of the aftermath of the September 11, 2001 terror attacks, when mail laced with anthrax appeared in post offices, newsrooms and U.S. congressional offices. Five people died and several others were made ill.
The FBI attributed the attack to a government scientist who killed himself in 2008.
Ricin previously turned up in a U.S. Senate mailroom in 2004, forcing authorities to temporarily shut down two Senate office buildings.
Senator Wicker's office had no immediate comment on this latest incident. The 61-year-old lawmaker has been in the Senate since 2007.
Some information for this report was provided by AP and Reuters.