I will need to review the episode later tonight. My memory of the shooting is that the gun was fired from the passenger side. I, also, remember that Holmes identified the gun as an AP-15. This is a carbine rifle. In this video, the shooter is using both hands to use the rifle.
Well, maybe they got their research wrong; it would hardly be the first time in the history of television. But Holmes did definitely say the gun could be fired one-handed, so the intent of the writers was that there was only one person in the car, even if the details don't mesh with reality.