BISMARCK, N.D. (KXNET) –The North Dakota Department of Corrections Monday morning reports convicted murderer Chad Isaak has died of “self harm,” according to the department.

“On Sunday, July 31, 2022, at 5:44 p.m., the North Dakota Highway Patrol was called to the North Dakota State Penitentiary, Bismarck, regarding a resident who had caused self-harm,” the department said in a news release. “The responding NDHP Trooper was told the inmate was taken by ambulance to Sanford Hospital where he was pronounced deceased at 6:24 p.m.”

The inmate was identified as Chad Isaak, 48.

The incident remains under investigation by the North Dakota Highway Patrol and the North Dakota Department of Corrections and Rehabilitation. No additional information is being released at this time.

In August 2021, Isaak was convicted of murder in the April 2019 deaths of Robert Fakler, Adam Fueher, Bill Cobb and Lois Cobb at the RJR Maintenance and Management building in Mandan.

In December 2021, he was sentenced to four life sentences without the possibility of parole.

On June 30, Isaak appealed his murder convictions to the North Dakota Supreme Court, arguing errors in his trial should prompt a reversal of his criminal convictions.