MANDAN, N.D. (AP) — A man accused of killing four people at a North Dakota property management company waived a preliminary hearing and pleaded not guilty to murder charges.

Chad Isaak’s waiver and plea was signed Friday by South Central District Judge Gail Hagerty. Isaak, a 45-year-old chiropractor from Washburn, is accused in the April 1 shooting and stabbing deaths of RJR Maintenance and Management owner Robert Fakler and employees Adam Fuehrer and Bill and Lois Cobb. The Cobbs were married.

Isaak has been jailed on $1 million bond since his arrest a few days after the shootings. He is charged with four counts of murder as well as lesser charges.

Isaak lived in a mobile home park managed by the company, but police have not identified a potential motive in the killings. Search warrants and receipts filed in the case show investigators have scanned Isaak’s bank and phone records, his home, office and vehicle and the Facebook accounts of the victims.

The four were known around the company as an informal “coffee club,” often arriving early to chat before the workday began. A police affidavit said the victims were shot and stabbed.

Deputy Chief Lori Flaten last week declined to answer questions about the case, citing prosecutors.