How is suspense created in the film you watched in class?

The film we watched in class was United 93.
The film created suspense by building up tension, one way this film did this was by using the technique "false plateau".
By using this technique it made us think that the danger had past, until the danger suddenly pops out and gets you by surprise, in this case, it was when the terrorist was starting their attack because we wasn't sure when they were going to do it.