Curtis wrote that he was trying to "expose various parties within the government, FBI, police departments" for what he believed was "a conspiracy to ruin my reputation in the community as well as an ongoing effort to break down the foundation I worked more than 20 years to build in the country music scene."
In one post, Curtis said he sent letters to Wicker and other politicians.
"I never heard a word from anyone. I even ran into Roger Wicker several different times while performing at special banquets and fundraisers in northeast, Mississippi but he seemed very nervous while speaking with me and would make a fast exit to the door when I engaged in conversation..."
He signed off: "This is Kevin Curtis & I approve this message."
The FBI said there was no indication of a connection between the letters and the Monday bombing in Boston that killed three people and injured more than 170. The letters to Obama and Wicker were postmarked April 8, before the marathon.
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 hearing, Postmaster General Patrick Donahoe noted there had been ricin alerts since the notorious 2001 anthrax mailings 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."
Associated Press writers contributing to this report from Washington were: Eileen Sullivan, Laurie Kellman, Donna Cassata, Henry Jackson and Eric Tucker.
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