Assault rifle? 9mm? Naww. Musta been a 9mm assault pistol :)
Remember that any weapon used in an illegal act is (as far as the media is concerned), an assualt XXXX, weather it's an "assault pistol," an "assault rifle," or whatever. These people aren't interesting in getting the details right, as long as the slant meets their political views.
BTW, there are a number of carbines out there that do fire 9mm. I believe that Savage makes one, and it uses S&W magazines. There's also the Colt 9mm version of the AR15, but I don't know if it's ever been offered in a semi-auto/civilian version. there are others that I've seen over the years, but I can't think of any right off hand.
Mark