The other thing I want to contribute is that the fuses were probably triggered by remote control. Radio fuses were available at that time. A high vantage point for the source of the radio waves is desirous so as to penetrate down the columns of the core of the building. There is evidence for "US military helicopters", as one witness describes it, circling the World Trade Centre in the first 15 minutes of this video:
Google video TM
Radio waves can not penetrate metal but can go around it. However, every time it does so the wave is scattered. Having several helicopters transmitting the same signal simultaneously from all around the building is ideal for an explosive demolition such as this type, by increasing the penetration and redundancy. If any part of the building failed to detonate properly there would have been a costly and hasty re-demolition effort in which the existing charges could have been discovered. Radio fuses have no wires to be threatened by other parts of the falling building so they're better, plus, they're a lot quicker to install! This explains why they only needed that two-day "power down", the weekend before, to install the charges in the World Trade Centre towers.
The above video shows a lot of helicopter activity. A perfect vantage point to perform your murder - with military precision, and a perfect get-away!