BISMARCK, N.D. (AP) — An investigation into the prison suicide of Chad Isaak found that he had covered his cell window with cardboard and a guard failed to follow proper procedures in conducting required checks.

The investigation report from the state Highway Patrol said Isaak was found hanging in his cell July 31. He was rushed to a Bismarck hospital, where he was pronounced dead.

Isaak was appealing his convictions and life sentences for the killings of four people at a Mandan property management company.

Sgt. Deandre Adams has been placed on administrative leave until an internal investigation at the prison is completed, prison spokeswoman Kayli Richards said Wednesday.

Adams declined comment when contacted Wednesday.

No charges against Adams are expected, said patrol spokesman Sgt. Wade Kadrmas.

The investigation by the patrol said Adams, who had worked at the prison for 15 months, said he failed to adequately check on Isaak twice at 4:53 p.m. and 5:28 p.m. The report said it was “unknown if Isaak could have been resuscitated” if he had been found during that time.

Adams knew inmates were not allowed to cover their cell windows, but allowed it, as a “courtesy in case they were naked in their cells,” the report said. “He stated they could enforce it but that they would just get more window coverings.”

Isaak was serving four life sentences without the possibility of parole for the April 2019 killings of RJR Maintenance and Management co-owner Robert Fakler, 52; and employees Adam Fuehrer, 42; Bill Cobb, 50; and Lois Cobb, 45. The Cobbs were married. No motive was given at the trial or established by authorities.

The four were shot and stabbed inside the property management company’s building in Mandan. Isaak, a chiropractor and Navy veteran, lived at a Washburn property that the company managed.

During the trial, defense attorneys argued that investigators didn’t seriously consider other possible suspects, including people who had either been evicted, sued or fired by RJR.

Prosecutors showed security camera footage from numerous businesses that authorities said tracked Isaak’s white pickup truck from Mandan to Washburn on the day of the killings, along with footage from a week earlier that they said indicated the killer had planned the attack in advance.

Forensic experts testified that fibers on the clothing of the slain workers matched fibers taken from Isaak’s clothing, and that DNA evidence found in Isaak’s truck was linked to Fakler and possibly Lois Cobb.