A letter addressed to U.S. President Barack Obama tested positive on Wednesday for ricin in a drama over poisoned mail spreading alarm in Washington, AFP reported.
The FBI said there was no connection between the letter sent to Obama and detected at a remote mail facility, and another mailed to Republican Senator Roger Wicker, and the 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, according to AFP.
The U.S. Secret Service said the letter to Obama 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.
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 U.S. 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 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.
It added, however, "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.
Adding to nervousness in the U.S. capital, two Senate office buildings were briefly cordoned off amid reports of a suspicious package but were reopened after the all-clear was given.
U.S. Capitol Police confirmed one man was being questioned.