Preliminary tests carried out by the FBI show that a letter addressed to President Barack Obama contained the deadly poison ricin.

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 is remote and not located near the White House itself, spokesman Edwin Donovan said in a statement.

"This facility routinely identifies letters or parcels that require secondary screening or scientific testing before delivery," Mr Donovan said.

The FBI said the envelope was immediately quarantined.

In a statement, it said: "The investigation into these letters remains ongoing, and more letters may still be received."

Yesterday, US authorities intercepted a letter sent to Mississippi Senator Roger Wicker that preliminary tests showed contained the deadly poison ricin.

Police say they are questioning an individual over the envelopes.

US law enforcement sources said there is no indication of a connection between the suspicious letters in Washington DC and Monday's attack in Boston.

The Secret Service is said to be working closely with the US Capitol Police and the FBI in the investigation.