LINCOLN, Neb. -- According to officials with the Nebraska Department of Correctional Services, 81-year-old Charles Kays has died at the Nebraska State Penitentiary.

According to an NDCS press release, Kays' sentence began on June 15, 2011.  He was serving a sentence of 16 years and eight months to 20 years on charges out of Douglas County that included first degree sexual assault on a child and two counts of third degree sexual assault on a child.

The cause of death has not yet been determined, NDCS officials said.

As is the case whenever an inmate dies in the custody of NDCS, a grand jury will conduct an investigation.