Volke

Best answer as chosen by Asker / Ratings
Head down the manhole in the Precinct Basement. After you reach a blocked path, Leon will help Ada over and you will assume control of Ada. Ada's path is fairly linear and you should find the final Precinct Key (the Club Key) after a few minutes.

Return to the area where Leon is and Ada will throw over the Club Key (and some Shotgun Shells if you grabbed them) before you assume control of Leon again.

You should have used the other three Precinct Keys (Heart, Club and Spade) and have witnessed Ben's death scene before you can actually access this area.