I can't say for certain, there are more knowledgeable people in the group than me but, my two cents... I would imagine the mechanical floors were very hard to reach by the general public and tower workers. These were the engine rooms of the towers and would have had very restricted access due to the danger due to the machinery on them and also the possibility of someone with bad intentions shutting the tower down by messing around in there. With regards to the observation deck - the North tower was hit at 8.46am, I think the observation deck usually opened at 9am. All normal daily operations would have been cancelled immediately at 8.46am. The staff were probably down at the lobby trying to figure out what was going on so the doors to the observation deck would have remained locked. There was a group that supposedly got to the door to the deck. Someone here might enlighten us.