It was apparently (and really, quite obviously) brought down by a controlled demolition.
There was not the time available to properly set up such a controlled demolition on 911. Not even close to the time available. No way they could have done it in the time available on 911.
If it had been a legit demolition operation done on 911 to protect lives and property we would have been introduced by now in the media to some of the professional "heroes", the explosives experts who did it and managed to pull it off successfully. We haven't been.