Have they for sure identified the gun(s) involved? I have a hard time believing a 9mm could be used to kill that many people. Plus the cell phone clip of the shots made them sound much deeper than a 9mm report.
Cell phone cameras have horrible sound recording capabilities, plus there had to have been a lot of echoes.
A 9mm *is* a major caliber and it is a killer; part of the reason the SAS loves their Browning High Powers. I suspect that most of those fatalities were due to blood loss and shock, though, not instant fatalities.