I remember watching the press conference with the M.E., in which he said that the injuries he observed were consistent with high velocity rifle rounds when asked about the weapon used. He then talked about firearms in a way that made be conclude that he's a shooter, because of the terminology he used and the way he described things. Don't ask for details, as I don't recall. Just left me with that impression.
As far as the time lapse goes, in my county (suburban/rural area of Central Ohio), average response time by law enforcement varies between 5-15 minutes, depending on where the incident occurred, the weather, the time of day, and a lot of luck (if a cruiser was in the area or on the other side of the county).