Here's my only issue with the idea of a revolver using a suppressor: the gap between the cylinder barrel. This gap still allows the expanding gas to escape which is where the sound of a gunshot comes from. A suppressor screwed into the end of a gun wouldn't account for this, so we should have heard a louder gun shot and not the typical "Hollywood" effect of the silenced discharge.

That means to me that the shooter had to police their brass. There is a gap between the shooter's first three shots and the final one. It could be during that time that the casings were picked up. And maybe they were able to pick up the last one directed at the radio as they were leaving.