THQ and Relic have made official rumours that Warhammer 40,000: Space Marine will sport a co-operative mode, but won't ship with it on-disc.
Kotaku reports the mode is called Exterminatus, and features up to four players battling waves of enemies.
The mode won't be released until early October, but will be free.
THQ accidentally let slip last month that co-op would be added post-launch, but delayed detailing it.
Space Marine is due for release on PC, PlayStation 3 and Xbox 360. It hits the US on September 6 with a European launch on September 9. A demo launched two weeks ago has proved remarkably popular.