The shooting happened about 10.30am on Friday at The Owl Bar in Anaconda, according to the Montana Division of Criminal Investigation, which is leading the investigation. The agency confirmed four people were pronounced dead at the scene.
The suspect, identified as 45-year-old Michael Paul Brown, lived next door, according to public records and bar owner David Gwerder.
The bartender and three patrons were killed, said Gwerder, who was not there at the time. He believed the four victims were the only ones present during the shooting, and was not aware of any prior conflicts between them and Brown.
"He knew everybody that was in that bar. I guarantee you that," Gwerder said.
"He didn't have any running dispute with any of them. I just think he snapped."
Brown's home was cleared by a SWAT team and he was last seen in the Stump Town area, just west of Anaconda, authorities said.
More than a dozen officers from local and state police converged on that area, locking it down so no one was allowed in or out. A helicopter also hovered over a nearby mountainside as officers moved among the trees, said Randy Clark, a retired police officer who lives there.
Brown was still believed to be armed, the Montana Highway Patrol said in a statement.
