The FBI confirmed the letters contained ricin Thursday, CNN reported.
However, the FBI said it was "not aware of any illness as a result of exposure to these letters."
Paul Kevin Curtis, 45, a resident of Corinth, Miss., has been charged with "knowingly depositing for conveyance in the mail and for delivery from any post office any letter, paper, writing or document containing threats to take the life of or to inflict bodily harm upon the president of the United States," a criminal complaint against Curtis stated.
He was also charged with sending "communications addressed to other persons, and containing a threat to injure the person of others."
During a hearing in federal court in Oxford, Miss., Magistrate S. Allan Alexander ordered Curtis to remain in custody until a preliminary hearing in the case April 29, which is also when a grand jury is expected to issue an indictment against him.
Christi McCoy, Curtis' attorney, told CNN his client "vehemently denies the allegations against him."
The letters sent to Obama, Sen. Roger Wicker, R-Miss., and Sadie Holland, a Justice Court judge in Lee County, Miss., were typed on yellow paper and stated: "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"