The FBI says further tests are still being done, but lab results show the toxin was used in the mailings. There were no known illnesses from the exposure.
Paul Kevin Curtis, the Mississippi man charged with mailing the letters, appeared in court Thursday and maintained his innocence. Curtis, 45, was arrested at his home in Corinth, Miss., at Wednesday.
Curtis wore shackles and a Johnny Cash T-shirt as he walked into a federal courtroom in Oxford, Miss. His handcuffs were taken off during the brief hearing.
He "maintains 100 percent that he did not do this," his attorney Chrisi R. McCoy said, adding that she knows him and his family, and that it is hard for her to believe the charges against him.
Investigators accuse Curtis of sending letters to Mr. Obama, Wicker and Sadie Holland, a justice of the peace in Lee County, Miss.
According to an FBI affidavit, the letters to the president and Wicker say:
"No one wanted to listen to me before.
There are still 'Missing Pieces'
Maybe I have your attention now
Even if that means someone must die.
This must stop.
To see a wrong and not expose it,
Is to become a silent partner to its continuance
I am KC and I approve this message."
Curtis is an Elvis impersonator who had written to officials in the past and consistently signed his letters "I am KC and I approve this message," according to an FBI affidavit.
He apparently believed that the government was covering up an alleged black market for the illegal sale of human body parts, the affidavit states, and he wrote about the allegations in an unpublished novel called "Missing Pieces," which he wrote about on his Facebook page and in the letters to government officials, the FBI said.
Ricin is a potentially fatal poison. It is distilled from the castor plant and converted to powder. It can poison a person through the skin, but is deadliest when it is inhaled.
If convicted, Curtis could face up to 15 years in prison, $500,000 in fines and three years of supervised release.
The Associated Press contributed to this report.