How the building collapsed??

By now we all know that the buildings collapsed because of explosives. The question we have to ask is how they managed to place all the explosives
without someone notice?
It probably took place a lot of work during the power-out.

I'm no expert on this and I don't know if this has been talked about before. I was looking at this movie:


And in a interview about 36 minutes in to it they say this:

"One of the towers had a major elevator that went all the way up to the top
that was malfunctioning, and had been malfunktioning for at least a month.
They have been having a lot of trouble with that"

If the elevator that went all the way to the top was not used for a mounth, it gave them alot of time to place explosives to the core !!??

I really hope that someone look into this!!

/ from a worried swedish citizen