In several letters to Wicker and other officials, Curtis said he was writing a novel about black market body parts called "Missing Pieces."
Curtis also had posted language similar to the letters on his Facebook page, the affidavit says.
The documents indicate Curtis had been distrustful of the government for years. In 2007, Curtis' ex-wife called police in Booneville, Miss., to report that her husband was extremely delusional, anti-government and felt the government was spying on him with drones.
Curtis was arrested Wednesday at his home in Corinth, near the Tennessee state line. He was being held in the Lafayette County jail in Oxford, Miss.
He believed he had uncovered a conspiracy to sell human body parts on the black market and claimed "various parties within the government" were trying to ruin his reputation.
Curtis had been living in Corinth, a city of about 14,000 in extreme northeastern Mississippi, since December, but local police had not had any contact with him prior to his arrest, Corinth Police Department Capt. Ralph Dance told The Associated Press on Thursday. Dance said the department aided the FBI during the arrest and that Curtis did not resist. Since Curtis arrived in the town, he had been living in "government housing," Dance said. He did not elaborate.
Police maintained a perimeter Thursday around Curtis' home, and federal investigators were expected to search the house later, said local officers on the scene who declined to be identified. Four men who appeared to be investigators were in the neighborhood to speak to neighbors. There didn't appear to be any hazardous-material crews, and no neighbors were evacuated.
The material discovered in a letter to Wicker has been confirmed through field testing and laboratory testing to contain ricin, Senate Sergeant-at-Arms Terrance Gainer said Thursday. The FBI has not yet reported the results of its own testing of materials sent to Wicker and to President Barack Obama.