The Sanitarium - Second Lab and Ventilation Ducts
After the cutscene in which the Proto kills the scientist, Kurtis is standing in the hallway outside the lab. Another inmate (11) approaches from the right. Kill him.
Then continue to the end of the hallway. Pry open the vent...
...duck, and scooch through. (Kurtis doesn't exactly crawl.)
Climb up into the higher duct, follow it to the end...
...and somersault into the lab shown in the cutscene.
At this point, you may notice 2 more inmates (12-13) in the hallway outside the window. They can't get in, so you can ignore them. (Skim these tactics, though, since they may come in handy later.)
If you want all kills, take a standing jump onto the table below the duct, pull up into the opening, and return through the ducts to the hallway.
Kurtis can't throw the Chirugai while crouching, but he can aim the Boran X while kneeling. So if the nearest enemy is too close to the duct opening and won't move away, press Duck again to go on one knee, equip the gun, and draw.
Or, move out and quickly equip the Chirugai. Then press Duck to activate the psychic shield. This will reduce incoming damage so you can fight more effectively in close quarters.
The other enemy takes a little longer to approach. So run around the corner to meet (and kill) him. Then return through the ducts to the LABORATORY.
Cross the LABORATORY, pry open another vent at floor level, and go through.
Again, climb into a higher duct and follow it to the next room.
Cutscene: From hiding, Kurtis sees the Proto-Nephilim feasting on another soldier. Perhaps sensing Kurtis's presence, the beast flees through an opening in the ceiling.
Drop down and pick up the Boran ammo (12).
NOTE: The game is fairly generous with ammo, but if you do run out, the clip near the fan will respawn, so you can't get stuck here.
You can't get past the moving fan and you can't climb the broken ladder. So stand back and shoot one of the yellow tanks to cause an explosion that destroys the ladder.
Now you can pull up onto the ledge. Throw the switch to turn off the fan...
...then duck under it.
Go through the duct at floor level on the left.
Follow the hall to the right to finish the level.
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