[Pio1980]

A politically powerful minority has been induced to forcefully insist that any sensible meaningful policy to limit supply, access, or features of firearms, to include military combat purpose (high capacity/ rapid fire, f.i.) are infringements of constitutional right. Consequently, ready access by legal means, or otherwise, is available to literally anyone sufficiently motivated. The Anders Brevik case example demonstrates that achieving a 100% result by common sense management is unlikely under the best of circumstances, but definitely improves general safety and security without ovebearing infringement of actual liberty in a modern representative environment.
Otherwise;
This shooting appears a "lone wolf" attack by someone with a grudge or a cause. Whether part of a "cybermovement" of some sort remains to be seen.