The FBI said there was no connection between blasts at the Boston marathon that killed three people on Monday and mailings sent to Obama, Republican Senator Roger Wicker and an unidentified Mississippi justice official.
Special agents on Wednesday afternoon arrested Paul Kevin Curtis, "the individual believed to be responsible for the mailings of the three letters sent through the US Postal Service," the bureau 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 Wicker, which never reached them, were signed "I am KC and I approve this message."
Curtis was arrested at his home in Corinth, Mississippi, according to the FBI.
After preliminary tests on the Obama letter showed traces of ricin, further examinations are to be carried out in 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 sent to Wicker 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 minuscule 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.
A filter at a second mail facility had tested positive for ricin, according to the FBI, which warned that more letters could be received.
But it added: "There is no indication of a connection to the attack in Boston."
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 staffer 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 an 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 Capitol Police officer told AFP.
Republican Senator Jim Risch said 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 mailings 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.