If you just happen to have the "security" company in your hip pocket, ..... kinda makes a difference .... also, on the excuse of working on the phone wiring, or the plumbing or anything at all to do with the building, there could have been armies of tradespeople all over the place and they would do what they do under the guise of re-routing wiring or checking for plumbing leaks and the office minions would be none the wiser. Some would speculate as to how much disruption of the building would need to have been done in order to rig it for demolition. Note that in the case of the towers, there were many floors with no offices at all and the demolition rigging ninjas would have a totally free space to do whatever needed to be done. However, speculation is rather fruitless, what we have as a sure thing is the total destruction of both towers after they fell straight down and at 64% of the acceleration of gravity. That fact alone is HUGE evidence!