US authorities have arrested a suspect over mailings laced with the deadly poison ricin to President Barack Obama and a senator, officials said.

"Today at approximately 5:15 pm (2215 GMT), FBI special agents arrested Paul Kevin Curtis, the individual believed to be responsible for the mailings of the three letters sent through the US Postal Service," the Justice Department said.

The letters "contained a granular substance that preliminarily tested positive for ricin," it added in a statement.

Earlier reports had said that the letters to Obama and Republican Senator Roger Wicker of Mississippi were signed "I am KC and I approve this message."

Curtis was arrested at his home in Corinth, Mississippi, the Justice Department statement said.

The US Secret Service said the letter to Obama had been intercepted at a mail screening site on Tuesday, the same day authorities said a letter was sent to Wicker that also showed traces of ricin.

Ricin -- a highly toxic protein found in castor beans -- can, when inhaled, cause respiratory problems. Ingested orally, it is lethal in even miniscule quantities.

Secret Service spokesman Edwin Donovan said the agency, which protects the president and his family, was working closely with the US Capitol Police and the FBI.

The FBI said "There is no indication of a connection to the attack in Boston."

The White House deflected inquiries on the incident to the FBI and the Secret Service.

Senator Carl Levin meanwhile issued a statement saying one of his staffers had discovered a "suspicious-looking letter" at a regional office in Michigan and handed it over to authorities for further investigation.

The person had no symptoms, but was being held in hospital overnight as a precaution, the senator said.

Adding to nervousness in the US capital, two Senate office buildings were briefly cordoned off amid reports of a suspicious package, but were reopened after the all-clear was given.

Republican Senator Jim Risch told AFP that everyone on Capitol Hill was perpetually as "vigilant as they can be" and that such scares had been part of life for many years.

The poisoned mail episodes recalled the mysterious series of letters laced with anthrax sent to lawmakers and some journalists, which killed five people and sickened 17 others, following the September 11 attacks in 2001.

Congressional mail has been screened off-site ever since.

Three Senate office buildings were shut in 2004 after tests found ricin in mail that had been sent to the Senate majority leader's office.

The biological agent was also sent to the White House and the Department of Transportation in November 2003. There were no injuries in those incidents.