By Holly Yan, Josh Campbell, CNN

(CNN) — Stymied by treacherous terrain and untamed wilderness, authorities are struggling to find an Army veteran suspected of killing four people at a Montana bar last week.

Investigators say Michael Paul Brown, 45, vanished Friday after gunning down a bartender and three patrons at The Owl Bar in Anaconda – a community of less than 10,000 people nestled between the dense forests and formidable mountains of western Montana.

“This is an unstable individual who walked in and murdered four people in cold blood – for no reason whatsoever,” Montana Attorney General Austin Knudsen said Sunday. “So there absolutely is concern for the public.”

Among the countless potential hideouts: Garrity Mountain , a popular hiking and camping site lined with cabins f

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