From ABC News:

Cetin’s mother helped to identify him from surveillance camera images released by police. The arrest warrant said Cetin allegedly used a Ruger 10-22 that had a 25-round magazine.

This just shows that even if all AR-15s were banned crazy people will just go to the next available gun. Anti-gunners will use these instances to say there is a need to ban more and more guns until they are all banned.