A letter sent to President Obama that has initially tested positive for ricin and was received at an off-site location away from the White House is being investigated as possibly linked to another mailing with ricin sent to a senator, federal officials say.
The FBI said Wednesday that the letter was immediately quarantined by Secret Service.
NBC News reported that officials believe both letters were sent by the same person. That suspect has not been identified.
"A letter addressed to the president was received at a remote screening facility containing a suspicious substance. We are working closely with Capitol Police and the FBI in this investigation," a Secret Service spokesman said.
The FBI said there was "no indication of a connection" to the Boston Marathon bombings.
White House spokesman Jay Carney said the president has been briefed on the mailing.
All White House mail goes through the remote screening facility, officials said. The letter was received Tuesday.
The mailing addressed to Sen. Roger Wicker, a Mississippi Republican, was also intercepted at a mail facility Tuesday and did not reach the Capitol. It was postmarked Memphis, Tenn.
The facility used to review all mail sent to Congress is temporarily shut down while a search is under way for the possibility of any other tainted letters.
The FBI warned that more letters may be received.
Ricin is a poison found in castor beans.
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