So I noticed, but my post preceded sika's transcript. My knowledge of this case is poor so bear with me.
Were all the victims shot through the offside rear door? Whilst it was opened or through its opened window? Whoever did the deed must have been an accomplished marksman to get three fairly accurate headshots on target in split seconds, before any of them had a chance to react... other than Tucker and Rolfe at the front turning their heads to the right to see what was happening if Tate on the rear seat was shot first. The rigid forward-pointing heads in the drawing are misleading, I think.
I need to carefully read/listen to all the authentic docs from the various experts. Sika has transcribed some of the trial transcripts from the narrated vids you uploaded and these are the most important imo. As with all these cases there's a lot of very misleading info in the public domain eg constant reference to a saw off shotgun.
Initially I thought the gun person had opened the door(s) to shoot (found closed) and couldn't understand why ice hadn't formed inside but I now know the rear passenger side window was smashed and from my initial quick skim of the expert evidence it seems each victim initially sustained 1 disabling shot through the broken window Tate's side ie rear passenger side. Not only would the gun person need to be a marksman but also have the psychology to kill on demand. Afaik Steele and Whomes were not marksmen, did not have a history of aggression/violence and neither were military/police trained.
I don't think the victims were high enough up the chain to involve any government organisation if indeed such things even happen in 'real' life. This leaves imo the following:
Leah Betts grieving father, Paul Betts, who was a retired police officer firearms trained and at the time run a clay pigeon shooting business. Apparently at one time the police suspected him and checked all his firearms. Leah died from taking in excess water following an ecstasy tablet. Afaik the murdered trio, Steele and Whomes traded in cannabis but Tucker was responsible for the doors at Racquels nightclub where Leah was the night she took the tablet.
The trio had upset someone higher up the chain who had military/police trained personnel on their payroll who was happy to execute the trio for the right fee. Or the gun person had been trained by some terrorist organisation eg IRA.
I guess it's possible Steele had the right contacts but I can't see the motive and I very much doubt he personally was capable practically or psychologically of carrying out the murders.
The shootings appear to have been carried out ambush style with military precision leaving no forensic evidence eg footprints/tyre tracks. Everything appears to have been perfect from the location ie next to a shooting range to disguise the sound of gunshot - the expert said the sound would be heard over a mile away to maybe even planning it down to the weather ie snow which melted away footprints and tyre tracks.