I’d tie a demon presence to the hijackers. Not to more normal citizens who perished.
All I know is from a couple of experiences it could very well have been spirits of those who had died. One of them was a dream of a phone call from a 29 year old healthy cousin telling me he had died. I awoke in the empty house quite startled because I never dream of death especially that of a family member. And when I did wake up the phone was ringing. It was my mom calling me to tell me that my cousin (the same one I dreamed about calling me only a moment before)had passed away. His death was unexpected and a shock to the whole family.