Fighting Frontier (1943)

Kit Russell, appointed special investigator, has infiltrated a local robber gang ostensibly led by Snap, but has found out that an unnamed local person is really the brains behind the outfit. Despite the disdain shown him by his girlfriend Jeannie, Kit stubbornly continues the ruse even after he and several members of the gang are caught and given a trial by a vigilante mob, and sentenced to death by hanging.