Answer:
In the shooting scene, you'd be able to collect the shell of the bullet and that would be one step to find out what type of gun the perpetrator used. You could go through nearby stores and look through their CCTV camera and find out what kind of car it was and maybe even catch a glimpse of the perpetrators face. However if the shooting was in a residential area you could go door to door looking for witnesses. To see if anybody heard or saw anything.
