As I mentioned below, these airlines were piloted at the end by the terrorists. They will have to have been trained airline pilots.

Then, there was the attack method, which was sophisticated and showed a great understanding of the building design.

From an article on the architect of the WTC:

"The structural system, deriving from the I.B.M. Building in Seattle, is impressively simple. The 208-foot wide facade is, in effect, a prefabricated steel lattice, with columns on 39-inch centers acting as wind bracing to resist all overturning forces; the central core takes only the gravity loads of the building. A very light, economical structure results by keeping the wind bracing in the most efficient place, the outside surface of the building, thus not transferring the forces through the floor membrane to the core, as in most curtain-wall structures. Office spaces will have no interior columns. In the upper floors there is as much as 40,000 square feet of office space per floor. The floor construction is of prefabricated trussed steel, only 33 inches in depth, that spans the full 60 feet to the core, and also acts as a diaphragm to stiffen the outside wall against lateral buckling forces from wind-load pressures."

The aircraft I saw hit the building literally punched through the center of the structure, with flame and debris coming out the opposite side. I can only assume the first attack was similar.

This accomplished what no fire alone could have done: physically stripping the structural integrity from a large section of the upper levels of the building. The remaining steel in these areas, despite fireproofing, was weakened by the fire until the top sections of the buildings collapsed onto the floors below, which were unable to stand the massive shock and collapsed themselves.

There were pilots and engineers involved in this. These weren't teenage kids with dynamite strapped around them.