Very good posts on both painting a picture of the probable situation and also the type of gun used.
The choice of gun probably shows that this was an event that was carefully thought out and planned by the shooter. He went for a high capacity gun with a lot of penetrating power. The FN five-seven’s tiny .22 cal projectile can cut through kevlar vests and also penetrate multiple victims. So as you said when shooting into the crowd each bullet was potentially able to cause multiple casualties. Because of the tiny projectile most people would not consider it a good gun for self defense; it was designed for penetrating body armor.
The point that I wanted to make is that the shooter’s choice of gun indicates that this was most likely a scenario that was well thought out. If he had just “cracked” as many commentators keep speculating he most likely would have picked one of the more commonly used calibers such as 9mm or .45 auto. The FN five-seven is a specialty gun.