Complex sentence: These shutters were shut during the game as they were at every game and the panicked persons moving down the enclosed stairways led to those in the lead being pressed against the closed steel shutters which were not visible to the panicked crowd pushing down the stairs from behind.
Split into simpler sentences: These shutters were shut during the game as they were at every game. Panicked spectators moving down the enclosed stairways pressed those in the lead against the closed shutters, but this was not visible to the crowd pushing down the stairs from behind.