Use the door left of O'Neal and go down the stairs. In one of the cardboard boxes you can find the above Dog Tag (Cpl C. Nelson).
As for the cells, you can locate them with the motion tracker. Two of them are easily reachable; you only need open the interactive door and shoot a single enemy (screenshot 1). Interact with the cells to disable them (screenshot 2).
Getting to the third cell is a little more problematic. Look for a stairwell pictured on screenshot 1; it's in one of the corners. When you reach the room upstairs, find the hole in the floor (screenshot 2). Depending on preferences, you can throw in a grenade or jump down to get rid of the enemies yourself. When everyone's dead, disable the third cell.