As the game progresses, you find that in the 17 weeks since the events of Aliens, Weyland-Yutani had set up a base of operations on LV-426 for top secret research. Several of these bases appeared to be overrun. We also see that Wey-Yu was breeding aliens in their facilities. Likely capturing people for their experiments. There were also hundreds of marines laying siege on the planet, most of which were captured or killed by the Aliens.
