>>515384879
Access to rooftops in places like these are often limited to one or two places and behind locked doors and/or padlocks to keep the rooftop hatch at the top of the ladder secured.
So it would have to be someone working there with access to those areas of the building, or someone who knows how to pick the locks... or brute force them open.
Either way, it's someone who knew Charlie was coming. Someone who knew exactly where the event was setup, had the ability to get to the rooftop unnoticed, and homicidal nature to actually go through with carrying out such a senseless act.