Some fat head, never identified, got on an intercom heard throughout both towers telling people there was nothing to be concerned about, and please remain in place. A friend of mine working on Floor 17 as a commodities trader looked out the window at the adjoining tower, realized if both were hit this was no accident, and left with a co-worker. One can hope the idiot who said to stay was not in league with the terrorists, and suffered the fate of those foolish enough to have listened to him.
Some people above where the planes hit could not get out due to fire below them. And of course many of the impacted floors were killed outright