Proceed towards the alley where the second guard has appeared for the first time. Eventually you should be able to get to a small entrance to this building [#205]. Go there now and use the stairs to find yourself standing on an upper balcony. Keep heading forward and turn left once you're inside the building again.
Once you're standing inside a new corridor press the crouch key and get to the left corridor carefully. You should notice a prisoner here, as well as a single guard [#206]. Approach the guard slowly and perform a stealth kill on him by snapping his neck. You can choose the louder execution, because his death won't alarm anyone.
Once the guard has been taken care of, talk to the inmate. Mosely will tell you about a possible way to get into the mines [#207]. As you've probably noticed by now, he's one of the blueskins. As a result kill him when you end the conversation. This can be done easily by standing behind him and performing a standard execution.
Go back to the junction and choose a door to your left. You'll have to be extremely careful here, because this entire corridor is being patrolled by one of the guards. Depending on your luck you may end up in front of him and unprepared, but on the other hand you may be given a chance to surprise him. If you're in a bad spot approach him as soon as possible and perform a countermove [#208]. Check his body for a much needed keycard.
Ignore the door at the end of this hall, because you'll be visiting that area later on. Instead focus on exploring an upper balcony [#209], the same one you've seen at the beginning of this mission. Wait for the guards to turn around before you enter the balcony. Turn right and quickly take a cigarette pack (40 - Upper Mines) from the ground. Go back inside before the guards decide to attack you.
You may now return to the corridor where you've killed a guard and one of the blueskins. There's a keypad nearby [#210]. Use your newly acquired keycard to unlock this passageway. Step inside and wait for the game to finish loading a new map.