I also got stuck here
IIRC when you are at the bottom of the stairs there is a compartment in the floor that can be open with a melee attack.
Thanks! There was a seal I could shoot so I could leave the building.
I just remember that last time I left the building through a window at the top floor, but then got stuck outside between some obstacles, hence I had to re-load the savegame, but couldn't reach the top floor anymore.
Anyway.. problem solved, thanks a lot.