Exit this room through the door and drop down to a flight of stairs, climb those and you will enter another part of the building and into the security office where you get a CS and find the armory here. A Security Guard shows up and points his weapon at you - this is another moral choice point - take his weapon or shoot him.
The Security Room
There really is no graceful end to this encounter - and after it resolves wither way you get a graphic CS and then you hear someone nearby begging for their life. Down a short hall and at a door you hear a merc explain to the hostage that he is about to execute that the man's name is on the list - not a good sign really - but hey, you gotta rescue him, right? With that done, a squad appears at the other end of the room for you to engage and kill - which you do - and then you move into an open area with more enemy units.
At this point your partner picks up a shield - you can choose to do the shield thing or branch out on your own and selectively prune this crop of merc's - that is what I did. Follow your partner through the nearby door and up the stairs - you should be approaching the level with Murray now. At the end of the hall is a security gate you co-op lift, and inside a short CS shows you her location.
Move in and take down the three merc's that are a direct threat to her, and rescue her. At this point there is a little dialogue but nothing important is said - and then another squad of merc's attacks. You will be a little busy taking them out - hopefully quickly - and then you move back towards that large atrium like room.
A lot of things happen at once now - Murray tells you that she will use the crane to lower you to the ground level, but as you enter the open room an enemy helicopter appears outside, and is hit by an RPG. The pilot loses control and crashes it into your building, and Murray yells to you that the helo cleared the way and you should make for it. There is a pair of heavy armor merc's on the stairs on the way down so go easy there.
Once you reach the floor below, move across the room towards where the helo crashed, taking out the bad guys as you progress, then enter the room there and drop through the floor, and then pause for a moment while Murray tells you what she knows. Once she is done filling you in, move through the last two rooms to the crane platform and Murray will lower you down. Her plan is to find someplace to bunker down and wait out the attack while you do what you do best.