SIOUX FALLS, S.D. (AP) — Authorities say a South Dakota state prison inmate serving a life sentence for killing a Highway Patrol trooper has died behind bars.

Corrections Department officials say 54-year-old Wade Aikins was found unresponsive in his cell Saturday at the South Dakota State Penitentiary in Sioux Falls. A preliminary investigation shows the death is due to accidental asphyxiation. An autopsy will be conducted.

Trooper Oren Hindman was transporting two people to jail in Spearfish in 1985 when Aikins stabbed him in the throat. Aiken was sentenced to life in prison after being convicted of second-degree murder.

He was also sentenced in 1994 to consecutive 5-year sentences from Bon Homme County for attempted escape and possession of a weapon by an inmate.