The longtime mayor of a small Mississippi town who once worked as a jail warden has been charged with sexually assaulting an inmate and trying to cover it up, authorities said Tuesday.

William Grady Sims, who's the mayor of Walnut Grove and the former warden of Walnut Grove Transition Center, was indicted on two federal charges last week. He appeared Tuesday in U.S. District Court in Jackson and was released on a $10,000 bond. A trial date for was set for Jan. 9.

The indictment alleges the mayor assaulted the inmate in November 2009. He's also charged with telling the inmate to lie to investigators in March 2010.

Sims was first elected in 1981 and is in his eighth term. He didn't immediately respond to messages left Tuesday at his office and home. A call to his attorney wasn't answered. The lawyer's voice mailbox was full.

U.S. Attorney John Dowdy and Daniel McMullen, the top FBI agent in Mississippi, announced Sims' indictment Tuesday in a news release, saying Sims was charged with sexual assault of an inmate while acting under color of law.

Court records only identified the inmate by initials and did not say whether the prisoner was male or female. The Walnut Grove Transition Center is privately run, but houses house men and women in the custody of the Mississippi Department of Corrections.

State Auditor Stacey Pickering announced Tuesday that Sims has been ordered to pay $31,530 for using city employees and equipment to work on private prisons in the area, including the one he ran.

"The demand issued against Mayor Sims represents multiple instances where city employees were directed by the mayor to do work at a private prison facilities located in Walnut Grove," Pickering said in a news release. "Taxpayers of Walnut Grove have been paying for equipment and labor to do work at these facilities that are for-profit, private prisons. In addition, town equipment and labor have been used on private property at taxpayer expense."

The Mississippi Department of Corrections has said the Walnut Grove Transition Center would "provide opportunities to reduce recidivism by offering job training and placement, as well as housing for qualified inmates."

Walnut Grove has a population of about 1,900. It is in the center of the state, about 65 miles northeast of Jackson.