I live near Jeffco. The local news has reports that several people were being extricated from the Cheyenne, but these news items are so sketchy right now that this could be wrong. The C172 burst into flames on impact after apparently losing a wing. I did not see this event, this is only from the local news media.
Flying out of Jeffco myself, and from the location (1/2 mile NW of the football field in Denver), that places the impact close to, or just outside, the perimeter of the Class Delta airspace at Jeffco. I'm unsure as to whether tower-to-tower clearances are available from BJC to APA, if so then the Cheyenne was much closer to BJC and may have been under that tower's control still.
This is all from what I have heard on the local news, and may be totally wrong. At the time of day (not long before 1800 local), and with mountain obscuration to the west hiding any sunlight left in the area, visual acquisition from the tower at that distance would be minimal, as far as seperation is concerned.
This is very bad. At least one house fully collapsed on fire when the C172 hit, but luckily no-one was home.
Bruce.
BJC, Jeffco, CO.
Edit: Latest TV news : Looking now at the impact site, it's further away from Jeffco than I first heard, and both aircraft were outside of Jeffco airspace (although I'm not sure if the Cheyenne was talking to Denver TRACON). The C 172 had not filed (VFR nor IFR), although was on a flight over 50 nm at night (APA-CYS). Altitude at impact was 7614', according the the FAA. The area is one popular for circumnavigating the Denver Class Bravo and the adjacent Jeffco Class Delta (for north-south flights). The direction of flight appears to be fractionally east of north (C172) and south (Cheyenne), so both may have been at 7,500' (although this is less than 3,000' AGL, so VFR rules don't necessarily apply).
Bruce.
BJC, Jeffco, CO