The FBI arrested a Mississippi man on Wednesday in connection with letters sent to US President Barack Obama and two other officials that are believed to have contained the deadly poison ricin, the US Justice Department said.

Paul Kevin Curtis, 45, was arrested at his home in Corinth, Mississippi, and is "believed to be responsible for the mailings of the three letters sent through the US Postal Service which contained a granular substance that preliminarily tested positive for ricin", the Justice Department said in a statement.

Authorities were waiting for definitive tests on intercepted letters that were addressed to Obama and Republican Senator Roger Wicker. Preliminary field tests can often show false positives for ricin.

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

Ricin is a lethal poison found naturally in castor beans, but it takes a deliberate act to convert it into a biological weapon. Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead. No known antidote exists.

The poison scare hit Washington after bombings at the Boston Marathon killed three people and injured 176 on Monday, but the FBI said there was no indication the incidents were connected.

Earlier on Wednesday, a flurry of reports about the letters and packages rattled the US capital and caused the temporary evacuation of parts of two Senate buildings.

Both letters said "To see a wrong and not expose it, is to become a silent partner to its continuance" and were signed "I am KC and I approve this message".

Two law enforcement sources said investigators believed the man arrested was the same Kevin Curtis who they say has posted rants on the Internet and performed as an entertainer and Elvis Presley impersonator.

In an online comment on an Elvis blog post in 2007, a Kevin Curtis complained that several Elvis contests "were rigged with hosts and judges getting kick-backs". The signature was: "This is Kevin Curtis and I approve this message."

For Washingtonians, the situation was an unsettling reminder of events of nearly 12 years ago, when letters containing anthrax spores were mailed to the Washington offices of two senators and to media outlets in New York and Florida, not long after the Sept 11, 2001, attacks in New York and Washington.

In Corinth, a city of about 14,000, police cordoned off part of a subdivision where Curtis lived. At least five police cars were on the scene, but there didn't appear to be any hazardous-material crews, and no neighbors were evacuated. The one-story, single-family home is similar to the others in the neighborhood, with red brick and white trim.

Ricky Curtis, who said he is Kevin Curtis' cousin, said the family was shocked by the news of the arrest. He described his cousin as a "super entertainer" who impersonated Elvis and numerous other singers.

"We're all in shock. I don't think anybody had a clue that this kind of stuff was weighing on his mind," Ricky Curtis said in a telephone interview.

AP - Reuters