You can reach the building that is destroyed by the helicopters in the end. Just get out of map , go on the bridge, from the bridge jump on the clip on the fence to the right , walk till you reach the buiding , jump from the clip to the building's last floor. There you can enter the room with mg's or go in some other near buildings . When the building gets destroyed you can get on the roof.
I'm too lazy to make a wmv so here's a screen
