“This is an unstable individual who walked in and murdered four people in cold blood for no reason whatsoever,” Montana Attorney General Austin Knudsen told a news conference on Sunday.
“So there absolutely is concern for the public.”
He added that officials had since widened their search area to include national forest land outside Anaconda, a town with a population of almost 10,000 in south-western Montana that is surrounded by dense, mountainous terrain.
“We want to find this guy,” Mr Knudsen said. “This is a dangerous individual who has committed an absolute heinous crime against this community.”
Around 250 law enforcement officers – federal, state and local – have been aiding in the search on foot as well as by car and helicopter, officials said.
The shooting happened at The Owl Bar at around 10:30 local time on Friday morning, officials said.
Mr Knudsen noted that the suspect was a regular at the pub and had lived next door. He said it was “likely that he [the suspect] knew the bartender and these patrons, which makes this even more heinous”.
Cassandra Dutra, another barmaid, told CNN that the suspect would come into The Owl Bar frequently, but “he wasn’t a part of the camaraderie” among customers. Ms Dutra was not working on Friday.
Ms Kelley’s daughter, Nancy Kelley, told NBC News her mother had been a nurse for around 30 years until retiring recently, when she found part-time work at the bar as “it kept her a little bit social, just seeing people”.
She said a mass shooting was once unthinkable in Anaconda.
“We didn’t even lock our cars outside, you know, or the house, and it’s, I mean, I wouldn’t say, it’s dangerous here at all,” she said.