The FBI said Paul Kevin Curtis, 45, of Mississippi was arrested on Wednesday.
The FBI said there was no indication of a connection between the letters and the Boston bombing. The letters were postmarked April 8, before the marathon.
Authorities still waited for definitive tests on the letters to Obama and Senator Roger Wicker of Mississippi. An FBI intelligence bulletin obtained by The Associated Press said those two letters were postmarked Memphis, Tennessee.
Both letters said: "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." Such wording is used by candidates at the end of campaign ads.
The letters were intercepted before reaching the White House or Senate. The FBI said more testing was under way. Preliminary field tests often can show false positives for ricin.
As authorities hurried to investigate three questionable packages discovered in Senate office buildings, reports of suspicious items also came in from at least three senators' offices in their home states.
Sen. Carl Levin said a staff member at his Saginaw, Michigan, office would spend the night in a hospital as a precaution after discovering a suspicious letter. The staff member had no symptoms, Levin said in a statement. He expected to learn preliminary results of tests on the letter by Thursday.
Sen. Jeff Flake of Arizona said suspicious letters at his Phoenix office had been cleared, with nothing dangerous found. A package at Sen. John Cornyn's Dallas-area office also was declared harmless, a fire department spokesman said.
All three packages in the Capitol complex turned out to be safe, Capitol police spokeswoman Makema Turner said late Wednesday. But a man was still being questioned after being stopped in connection with the packages, she said.
Tensions have been high following Monday's bombings at the Boston Marathon that killed three people and injured more than 170.
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.
At a House of Representatives hearing, Postmaster General Patrick Donahoe noted there had been ricin alerts since the notorious anthrax mailings in 2001, and procedures are in place to protect postal employees and help track down culprits.
"Over the course of years we've had some situations where there have been ricin scares," Donahoe said. "Until this date, there's never been any actually proved that have gone through the system."
After the hearing, Donahoe said he didn't know whether the latest letters had been proven to contain ricin. No postal workers have reported illness connected to the incident, he said.
Ricin, derived from the castor plant, is at its deadliest when inhaled.