I uploaded screenshots of my movement here:
 http://mohaahelp.host.sk/mohaa1.jpg   (stairs to the room with 1st paper check) 
http://mohaahelp.host.sk/mohaa2.jpg   (paper check) 
http://mohaahelp.host.sk/mohaa3.jpg   (look from the stairs in the "big room" to the right) 
http://mohaahelp.host.sk/mohaa4.jpg   (the door from the "big room" on the right side from the stairs) 
http://mohaahelp.host.sk/mohaa5.jpg    (the door from the "big room" on the left side from the stairs - after killing the suspicious officer) 
http://mohaahelp.host.sk/mohaa6.jpg   (the door from the same room as mohaa5.jpg on the right side behind the shelves)
please tell me which doors do you use to proceed - the one on mohaa4.jpg, mohaa5.jpg or mohaa6.jpg - because I tried to open all of them and they were all closed...  
 
maybe I am really blind....