The man was arrested Wednesday at approximately 5.15 pm local time, Fox News reported citing FBI Special Agent in Charge Daniel McMullen.
Discovery of the deadly letters addressed to Obama and Republican senator Roger Wicker Tuesday, just a day after the Boston bombings, had rattled Washington.
The letter to Obama was intercepted at an off-site White House mail facility and apparently was from the same sender who mailed a "very similar" letter to Wicker, the Federal Bureau of Investigation (FBI) said Wednesday.
The Department of Justice said Wednesday the suspect was arrested at his Corinth, Mississippi, home following an investigation conducted by the FBI Joint Terrorism Task Forces in Memphis, Tennessee and Jackson, Mississippi, the US Capitol Police, and the US Postal Service, the US Secret Service.
The suspect has been identified as 45-year-old Paul Kenneth Curtis of Tupelo, Mississippi, Fox News said citing an unnamed source. Tupelo is also the hometown of Wicker and his wife.
Fox News citing an FBI bulletin said the letter sent to Obama and Wicker included the phrase: "To see a wrong and not expose it, is to become a silent partner to its continuance."
Both were signed, "I am KC and I approve this message." The envelopes had Tennessee postmarks, but no return address.
Obama's press secretary Jay Carney said mail sent to the White House is screened at a remote site for the safety of the recipients and the general public. He declined to comment on the significance of the preliminary ricin result, referring questions to the FBI.
Government laboratories are also testing samples of a suspicious substance found in letters at off-site White House and Senate mailrooms after preliminary test results pointed to the deadly poison ricin.
Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms-an amount the size of the head of a pin-can kill an adult. There is no specific test for exposure and no antidote once exposed.
The FBI said Wednesday it has no indication of a connection between the tainted letters and Monday's bombings at the finish line of the Boston Marathon that killed three people and wounded 183 others, according to latest reports.
Meanwhile, the Democratic chairman of the Senate Armed Services Committee Carl Levin said one of his Michigan home-state offices received a "suspicious-looking" letter and alerted authorities. "We do not know yet if the mail presented a threat," he said.
Mail for members of Congress and the White House has been handled at off-site postal facilities since the 2001 anthrax attacks.