Rapid City Police are investigating a beating in downtown Rapid City that left a 38-year old man with serious injuries. The incident happened Monday night around 6:30 when a passerby saw the three men assaulting another man. Leon Long Soldier was taken to Rapid City Regional Hospital where he's listed in stable condition Tuesday night.

Three men were arrested for aggravated assault. 32-year old Lowell Beaudreau, 30-year old Waylon Hill and 37-year Daniel Tail, all of Rapid City.

Police say they're not sure what started the fight but do know all the men knew each other.

Justin Pazera