The U.S. Secret Service says a letter addressed to President Barack Obama containing a "suspicious" substance was intercepted at a mail screening facility outside the White House.
The letter was discovered at the remote facility used to screen White House mail on Tuesday, the same day authorities said a separate letter sent to Republican Senator Roger Wicker was found to have been laced with ricin, a potentially fatal natural toxin.
The FBI said additional testing is needed as field and preliminary tests produce inconsistent results. The U.S. Capitol Police said more tests were being conducted.
The letter sent to Senator Wicker tested positive for ricin in three separate analyses. 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.
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.