Independent developer and publisher Tripwire Interactive has sold 1.1 million copies of Killing Floor
since the standalone version of the co-op first-person shooter released for PC in 2009.
The studio says the game, which began as an Unreal Tournament 2004
mod that debuted in 2005, is still selling strongly despite its age. Georgia-based Tripwire has also sold nearly 2 million downloadable content packs for Killing Floor
Tripwire's PC games, which include the Red Orchestra
series, have sold more than 2 million copies so far since the company was founded in 2005. It will release Rising Storm
later this year, and has begun publishing externally developed titles like ACE Team's Zeno Clash