The question raised over the "slipping" of the aircraft into the second tower is interesting. I have watched the approach of the UA 767 into the building and concluded:
1. The pilot crossed his controls to expose the wings to 5-10 floors, as opposed to crashing in into one, to gain maximum devastation.
2. It was a higly executed and professional flying manouver to precisely hit the spot and to prevent any possible drift - which suggests complete manual control over the aircraft.
It is also interesting to note that at least one of the aircraft, before the transponder was turned off, indicated a new airport code had been entered into the FMC which suggests the hijackers had enough knowledge to re-do the route. This would indicate a relatively high level of workings in the systems and handling of the aircraft - something that would surely come from groundschool and sim preparations.
A shocking, deeply disturbing episode in world history.
May God give wisdom to those left, and blessings to those lost.