A letter addressed to President Barack Obama carrying traces of a suspicious substance was received at a White House mail screening facility on Tuesday and preliminary tests showed it contained the deadly poison ricin, the FBI said on Wednesday.
An FBI statement said that an envelope addressed to the president and "containing a granular substance that preliminarily tested positive for ricin" had been received.
The facility where the letter was received is remote and is not near the White House, Secret Service spokesman Edwin Donovan said in a statement.
"This facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery," he said.
"The Secret Service is working closely with the U.S. Capitol Police and the FBI in this investigation."
On Tuesday U.S. authorities had intercepted a letter sent to Mississippi Senator Roger Wicker that preliminary tests showed contained ricin.
White House spokesman Jay Carney said Obama had been briefed on the situation involving the letter. He said that, according to the FBI, there was no indication of any connection between the letter and the Boston Marathon bombings on Monday.