U.S. congressional officials say a letter sent to Republican Senator Roger Wicker 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 say it tested positive for ricin in three separate tests. The Senate's sergeant at arms, the chamber's chief law enforcement and administrative official, said the letter was postmarked in Memphis, Tennessee, located near the Mississippi border.
Wicker issued a brief statement thanking law enforcement for "their hard work and diligence" in keeping lawmakers and their staffs safe.
The U.S. Capitol Police says it has launched a joint investigation with the FBI into the incident.
Ricin previously turned up in a U.S. Senate mailroom in 2004, forcing authorities to temporarily shut down two Senate office buildings. Ricin is highly poisonous -- just a tiny drop is lethal for adults.
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.