Level 2 Problem

After Killing the guards on the ground, and going to the second floor via the box inside the building, I sniper is no longer up there and I can't exit this area. How do you get past the Gate at the bottom of the stairs?
I tried to jump from the second floor but could not get to the wall left of the upper level.

Try going to where the sniper was and look for a passage or a staircase to your left.