A letter addressed to US President Barack Obama tested positive for ricin, spreading alarm in a drama over poisoned mail Wednesday in Washington -- which was already on edge after the Boston bombings.

The FBI said there was no connection between the item sent to Obama, which was detected at a remote mail facility, and another sent to Republican Senator Roger Wicker, and blasts at the Boston marathon which killed three people.

After preliminary tests on the Obama letter showed traces of ricin, further examinations will be carried out over the next 24 to 48 hours, the FBI 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 to trace the origins of the letter.

NBC News cited unidentified government sources as saying that officials believed they knew who sent the letters, reportedly with a Tennessee postmark, but that no arrests had been made pending the further tests.

The FBI said in a statement that a filter at a second mail facility had tested positive for ricin, and warned that more letters could be received.

But it added: "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.

US Capitol Police confirmed one man was being questioned.

"Right now they are interviewing a person but that person is not in custody. He has not been detained," a US Capitol Police officer told AFP.

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.