Valve has announced that Killing Floor, a first-person co-op survival horror developed by Tripwire Interactive, is now available for pre-order via Steam. Killing Floor is set in and around London after a mad scientist's "specimens" have broken loose and gone on the rampage. Killing Floor includes a co-op game mode for up to six players obliterating multiple waves of specimens, a solo game mode for offline play, 9 different monster types, a persistent Perks system, full support for Steamworks features, and more. Killing Floor is now available for 25% off its regular price for those who pre-purchase the game before it is made available for play on May 14. Check the new screenshots uploaded in our gallery.