US authorities have arrested a suspect from Mississippi in connection with a letter that tested positive for the poison ricin that was sent to President Barack Obama, a law enforcement source has said.

Secret Service spokesman Ed Donovan said on Wednesday that the letter was intercepted at a facility away from the White House, adding that the letter was received on Tuesday.

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

"The Secret Service is working closely with the US Capitol Police and the FBI in this investigation."

The FBI said late on Wednesday it had arrested Paul Kevin Curtis, of Corinth, Mississippi, in connection with the letters.

The Federal Bureau of Investigation said preliminary tests on a letter sent to President Barack Obama indicated the presence of ricin.

But the FBI statement added: "There is no indication of a connection to the attack in Boston," where three people were killed in bombings at the Boston Marathon on Monday.

The letter is undergoing further testing because preliminary field tests can be unreliable, creating false positives.

Al Jazeera's Patty Culhane, reporing from Washington DC, said: "It will take up to 48 hours for them to find out if it is ricin."

Senator targeted

It came after legislators said a different letter was mailed to Mississippi Senator Roger Wicker that tested positive for ricin.

A law enforcement official, speaking on condition of anonymity, said the letter to Obama was very similar to the one mailed to Wicker.

Michigan Senator Carl Levin has also said his regional office in his state received a suspicious letter and that authorities have been alerted.

Levin said in a statement that an aide received the letter on Wednesday, but did not open it. Authorities are now investigating.

The Democratic legislator said he and his staff do not know if the mail presented a threat.

The episode also recalled the mysterious series of letters laced with anthrax that were sent to lawmakers and some journalists following the September 11 attacks in 2001 which killed five people and sickened 17 others.

Tensions have been high in Washington and across the country since the deadly bombings on Monday at the Boston Marathon that killed three people and injured more than 170.