I believe that the building had places to place explosives to bring it down; that, in fact, most if not all new high rises have this. However, I believe that the explosives are only placed close to the time that they want the buildings brought down. I have heard that explosives don't last that long and ofcourse there is the issue that if the explosives are in the building it makes it that much easier to blow it up in an inoportune time.
As to when the explosives were placed in the building, here's a good page:
http://www.prisonplanet.tv/articles/april2004/042304explosivesplaced.htm
Obviously some explosives "sweat" and there is the chance of destabilisation like you say, so that does narrow the time margin like you put. Of course it doesn't rule it out, the only thing that speaks against it is the loss of life. My use of Collateral Damage does of course mean the surround buildings and potential loss of life outside of the towers.