TECUMSEH, Neb. (AP) — A Nebraska inmate serving a sentence for sexually assaulting a child has died.

The Nebraska Department of Correctional Services says in a news release that 55-year-old Robert Nelson died Tuesday. He was serving a 60-year sentence at the state prison in Tecumseh for first-degree sexual assault of a child.

The cause of death hasn’t been determined, but Nelson was being treated for a medical condition.

He began his prison term in January 2014.

A grand jury will investigate his death. That is the policy whenever an inmate dies in prison.