[GSlob]

Same problem - shooting from inside the car would require swinging the bulky firearm around and bringing it towards the window - an awkward and time consuming proposition for everything with a non-collapsed shoulder stock. The policeman would have an ample time to react and draw his sidearm. And IIRC, assault rifles have projectile energy >1000 ft-lbs, while even a hot 9mm round from a moderate length barrel would not produce more than 600, and normally less.
That's why I'd think the perp was using a regular 9mm pistol, and assault rifle nomenclature comes from the reporter's gross ignorance.

[ASOC]

The perp did use a pistol, the comment was sent as a 9mm assault rifle was seen as not possible.

To a reporter - not well trained in much of anything dealing with weapons (or reality for that matter) a 9mm Colt would look just like an M-16 - a known assault rifle.

As noted in the other posts above, I agree an assault rifle would use an intermediate cartridge, be select fire, have a barrel of under 16 inches, large removable magazine and so on.