A letter addressed to President Barack Obama contained a substance that preliminarily tested positive for the deadly poison ricin, authorities said today.

News that the letter to Obama was being investigated came as a flurry of other reports of suspicious letters and a package caused the temporary evacuation of parts of two Senate buildings and set nerves on edge in Washington.

Law enforcement authorities reopened the Hart and Russell Senate buildings near the U.S. Capitol after tests on various items showed there was no threat.

"All test results were negative," U.S. Capitol Police said over the public address system in Senate office buildings.

The suspicious letter to Obama contained "a granular substance that preliminarily tested positive for ricin," an FBI statement said. It added: "There is no indication of a connection to the attack in Boston," where three people were killed in bombings at the city's marathon on Monday.

The Secret Service said the letter to Obama was received on Tuesday at a mail screening facility not located near the White House.

"The Secret Service is working closely with the U.S. Capitol Police and the FBI in this investigation," Secret Service spokesman Edwin Donovan said in a statement.

The FBI said filters at a second government mail screening facility had "preliminarily tested positive for ricin this morning" and mail from that facility was also being tested.

The tests were being conducted at Fort Detrick, in Frederick, Maryland, a government source said.

White House spokesman Jay Carney said Obama had been briefed on the situation.

Capitol Police earlier investigated a suspicious package delivered to Senator Richard Shelby's office in the Russell building, but had determined there was no threat, said Jonathan Graffeo, a spokesman for Shelby. "Senator Shelby and staff are unharmed," he said by email.

Senator Carl Levin said one of his Michigan regional offices had also received a suspicious-looking letter, but it was not opened. Authorities are investigating, Levin said.

On Tuesday, authorities intercepted a letter sent to Mississippi Senator Roger Wicker that preliminary tests showed contained ricin.

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