A 2003 fire in the 35-story Cook County (Chicago) Administration Building was typical of such tragedies, with locked stairwell doors that prevented people from re-entering building floors when they encountered heavy smoke in a stairwell.
In high-rise offices, it is a common practice to lock stairwell doors to prevent criminals from entering offices, committing theft and leaving via the stairwells. Sometimes doors are locked only at the end of a business day; but in more buildings today, doors are always locked above the first floor.