The bodies were found by police officers responding to a medical call at RJR Maintenance and Management in Mandan, which is located just across the Missouri River from Bismarck.
A news release from the Mandan Police Department stated officers found “several people who were deceased inside” the facility around 7:30 a.m.
It was revealed that the bodies of three men and one woman were discovered inside the building. Police said the bodies have not yet been positively identified.
The state Bureau of Criminal Investigation told The Associated Press it was helping with the investigation but did not offer any details.
Darin Helbling, a manager at a nearby bowling alley, said police asked to see his business' surveillance video. He said the video showed only a couple of vehicles on the road that separates the businesses since 10 p.m. Sunday.
Fox News' Faith Mangan and The Associated Press contributed to this report.