BISMARCK, N.D. (AP) — A man sentenced to life in prison for killing four people at a Mandan business in what authorities say was one of the most heinous crimes in North Dakota history plans to appeal his conviction.

A notice of appeal filed Thursday by Chad Isaak, 47, said Isaak is questioning whether the evidence in the case was sufficient to support his conviction.

A jury in August found Isaak guilty of killing RJR Maintenance and Management co-owner Robert Fakler, 52; and employees Adam Fuehrer, 42; Bill Cobb, 50; and his wife, Lois Cobb, 45.

The four were shot and stabbed on April 1, 2019, inside the property management company’s building in Mandan. Investigators said the victims were stabbed more than 100 times, total.

Documents filed with the North Dakota Supreme Court by attorney Kiara Kraus-Parr, of Grand Forks, provide no details on the exact argument that will be presented to the panel, The Bismarck Tribune reported

 Prosecutors presented extensive DNA and fiber evidence and referred to a cache of items in Isaak’s home as his trophies from the killings. Defense attorneys argued that the case was one of “confirmation bias,” meaning investigators determined Isaak was the killer and didn’t follow other leads that might have shown otherwise.

A clear motive has not been established for the killings.

Isaak had 30 days from the Dec. 28 sentencing to file a notice that he intended to appeal.