After meeting with the chief, you will need to return to your desk, found in the corner. Winterson's phone will ring, but the caller will hang up. You will have to meet with Winterson to let her know, and she is in the interogation room with a witness to the shootout in the previous chapter.
Wait for the police lineup to finish, and speak with the witness. Return to Winterson, and after the short discussion, Mona will have been returned to lockup.
Head downstairs, and follow the path to the lockup. The police will let you in to take a statement from Mona. In turn, she will mention that you need to let her out, and tells you to make a call. When you exit the holding cell, place the call, and try returning to your desk, the station gets attacked. The cleaners took the downstairs entrance, and try finding Mona.
Collect the two painkillers and weapon from the opened gun cabinet, and re-enter the holding area. Two cleanres will approach from the right, but you need to reach the police garage to the left. The garage has two vans of cleaners, which are easily dispatched. Head out to the street to finish the prologue, and to be picked up by Vladimir.