Ah now thats a can of worms!
The 412 is that of the NYPD who had plans to repeat the relatively large scale operations of the previous attack on the Twin Towers. No great numbers involved, under 100 shuttled to and from in a rescue and fire fighting roles but high profile and good
PR. It showed what might be achieved in the future despite the aerials and other bits making roof landings dicey.
Unfortunately they upset many of their opponents first time out - mainly the NY Fire Department and their supporters in high places - that resulted in the doors on to the roof being firmly locked against a repeat performance. There was and still is friction between the emergency services over this very aspect of 911 although they are reportedly at least more flexible about roof doors now.
There were a lot of floors above the 911 aircraft strikes - forcing some, perhaps many, upwards ..........
It does not bear thinking about too long...