It is a remake of the "Alien Swarm" mod for "Unreal Tournament 2004", and it was developed by the original team, who were hired by Valve during the course of the development process.
Игра является ремейком модификации Alien Swarm для Unreal Tournament 2004, созданного командой разработчиков, которые впоследствии присоединились к Valve.