I've read that the murder weapon was an AR-15 pistol modified with an arm brace. An arm brace looks a bit like a short shoulder stock with a strap to wrap around the forearm and steady the pistol. The AR-15 pistol in my opinion is very difficult to shoot straight and too bulky.
Another issue is that for a while the ATF ruled that arm braces mounted on a handgun converted it to a short barreled rifle which is illegal to possess without ATF authorization and payment of the $200 tax. The ATF recently reversed their ruling and decided that at least some of the arm braces were legal.
But legal or not, if the arm brace is used as a shoulder stock, then the shooter needs that tax stamp to remain legal.
I'm going to go out on a limb and suggest that the shooter was using the arm brace as a shoulder stock. This would make the firearm illegally possessed. If he did not use it as a shoulder stock, this means a handgun was used as the case with most firearm related murder in the USA.
Not that any of this matters to the victims. It will be brought up in arguments concerning laws broken (if any) prior to the shooting and the type of firearm used to do so.
Ranb