I have fired the M-21 (with suppressor) and the M-24. Most .223 rounds cannot be silenced (unless you get special subsonic rounds). There were "reports" according to the reports, I doubt if he is using a suppressor. He/They clearly can shoot ... as far as I can tell it still could be any weapon.
Most of the time you cannot determine the direction of origin of a single shot. The sonic boom of the supersonic projectile will echo off of buildings or trees along the way, causing even more confusion as to direction of the shooter.
I also find the choice of cartridge interesting. There are many hunting rifles in various calibers available that would do the job. Yet, just about the smallest size practical is being used. The .223 is light, small, and gives a mild report and little muzzle flash. I haven't seen anything about exactly what weapon is being used, but I would suspect a T/C Contender with a scope mounted. This is a single shot pistol with interchangeable barrels available in more calibers than I've heard of. Overall length would be under 18" and with a quickly detachable bipod would be capable of this kind of accuracy. One could take a shot then toss it under a vehicle's seat.
I would also guess there is an accomplice serving as lookout and wheel-man. Since there seems to be no specific targets, all the shooter needs is nobody nearby to witness him and anybody in open view several hundred yards away.
Just my two or three cents, anyway.