I heard that atleast 1/3 of the both towers were able to evacuate before they came down. At best survivors of those towers were from 60-70th floor as ive understood. Most of those "just in time" survivors are in hospital right now, 1700 of them, 200 in critical condition.
Whats very sad is, that the only survivors found from the site have been those few cops and firemen 30 hours ago.