When you must infiltrate the CIA complex to find a mole, an alternate entry route into the building is to sneak around to the ventilation systems as usual. However, instead of climbing the fence and entering through the disabled fan, sneak by the two guards near the van talking to the civilian by following the darkened front wall area near them. You will soon reach a drainage pipe going up the wall. Quietly scale this pipe and get onto the large outcropping above the van. In the crouched position, walk to an open vent in the front of the building and enter. You will need to open a trap door and drop down on the inside of the CIA building on the other side of the front doors. Stay up on top of the doors and wait for the guard to appear. Then, drop down on him and knock him out. Drag his body to the darkened area off to your left then follow that wall to the small counter where a guard stands. Stand up at the left edge of the counter near the wall and pick up the pop can. He should not notice you if you are quiet and still in the dark. Throw the can backward into the general area from where you started off. The guard will walk towards the sound and you may sneak by him. Go through the metal detectors. Enter the room on the right for some health and Sticky Shockers, or go directly into the other set of metal detectors to get a checkpoint. This will put you past two guards, a maintenance man, and a camera, which you would have had to pass if you had chosen to go through the ventilation.

Use the following route instead of going through the ventilation system. When you first start this level, go to the cage the has the fan in it. Do not jump the fence. Go to the right of it and turn on your night vision goggles. You should see a pole walk to it. Be extra quiet, as there are three guards outside a van talking. Climb the pole, and you should see a vent open. Go through it, turn left, and open the trap hole. Go through this. It will take you straight to the main hall. You will land on a bookcase. Go straight (or to the left side of the hall). Go straight to the main counter. Quietly jump the counter and knock out the guard. Go through the door and knock out the guard in this room. Go out the door to the right. Take a quick left and go through the inactive metal detector.