LINCOLN, Neb. -- A man who was serving time for a sexual assault conviction in Madison County has been pronounced dead.

Officials said 75-year-old Johnny Hernandez died on Feb. 17 around 5:00 p.m. at the Reception and Treatment Center in Lincoln.

According to authorities, Hernandez was serving a sentence of 25 to 50 years for charges of first-degree sexual assault in Madison County. His sentence had started on Aug. 20, 1999.

Officials said they have not determined the cause of death but Hernandez was reportedly being treated for a medical condition before his death. A grand jury will be doing an investigation.