Washington: A letter addressed to US President Barack Obama has preliminarily tested positive for the deadly poison ricin, the Federal Bureau of Investigation said Wednesday.
In a statement, the FBI said the investigation into the letter sent to Obama and another sent to Senator Roger Wicker was ongoing, adding there was "no indication of a connection" to the Boston Marathon bombings.
The FBI said additional tests would be carried out over the next 24 to 48 hours to confirm the presence of ricin.
The US Secret Service said the letter had been intercepted at a mail screening facility outside the White House on Tuesday, the same day authorities said a letter was sent to Wicker that also showed traces of ricin.
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.
At the US Capitol, sections of two Senate office buildings were briefly cordoned off amid reports of a suspicious package.
Capitol Police later said results of tests conducted at the Hart Senate office building were negative and the closed-off areas were reopened.
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.
Senator Carl Levin issued a statement Wednesday 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 discovery of the letters rattled nerves following the bomb attacks near the finish line at the Boston Marathon on Monday that killed three people and injured more than 180 others, though it was not clear if the incidents were linked.
The episodes 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.
Congressional mail has been screened off-site since the 2001 incident.
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.
Ricin, when inhaled, can cause respiratory problems. Ingested orally, the protein is lethal in even miniscule quantities.