Killing Floor was originally a total conversion mod for the game Unreal Tournament 2004, first released in 2005.The lead developer Alex Quick was first approached by Tripwire Interactive about porting Killing Floor to their game Red Orchestra: Ostfront 41-45, but Quick did not follow through. A few years later, when their user base started to dwindle, Quick contacted Tripwire again to negotiate the port to Red Orchestra, and its distribution via Steam—similar to what Tripwire had already done with another Red Orchestra mod Mare Nostrum—and then later, the retail game.The mod version had its fifth and final official update in July 2008.