LINCOLN, Neb. (AP) — Nebraska prison officials say a man serving a decades-long sentence for manslaughter has died in custody.

A news release from the Nebraska Department of Correctional Services says 51-year-old Milton Felder died Thursday night at the Nebraska State Penitentiary in Lincoln. Felder, who began his sentence last July, was serving a 36- to 40-year term for manslaughter and a weapons count in the 2017 shooting death of 21-year-old Brandon White in a botched robbery outside a north Omaha middle school.

Officials say Felder’s cause of death has not been determined, but say he was being treated for a long-term medical condition. Under state law, a grand jury is convened to investigate any time a person dies in police or state custody.