Find all of the ID Tags you get from the security guards. You
can eliminate them with the pistol. They will drop 9mm clips 
if you need ammunition. Once you have found all of the tags, 
find the battery in the blue lit room with some alien warriors.
Use the shotgun to dispose of them, then return to the room 
where the security guards are. Power up the left one, then the
battery console. It will not say "Lift Activated", but when you
go into the chestburster room it will display. Return to the 
beginning of the level where you encountered the first warrior.
You will see a lot of things. Shoot the facehuggers down with 
the pistol or flamethrower. You can pick up a flamethrower 
canister, pulse rifle, TAG, shotgun, and armor.