It took them ... oh, a couple of weeks, at least, maybe a month ... to do all the filming. Each day I went to work, the police cars had more holes in them, the building was missing more windows, and had more smudge marks. They had a guard at the building to keep people away... off the grass, especially; he said there were pyrotechnic squibs buried in the grass for when Arhnoldt shot in that direction.
Out of line of what you see on the movie screen are propane tanks, and thin copper tubes to all the cars that are supposed to be on fire. When they're filming, they toss a road flare into those cars then turn on the gas.
All the filming was done at night. I never saw any of the actors. During the day, it was just that one guard.