Law enforcement officers are scouring a mountainous area of western Montana for a military veteran who authorities say opened fire at a bar, killing four people.

Police have found the getaway car of a US military veteran who walked into a bar in the western state of Montana on Friday morning and fatally shot four people, but the gunman remains at large.

A manhunt was underway in Montana on Saturday for 45-year-old Michael Paul Brown, a former soldier accused of fatally shooting four people at a bar.