Young, male muslim. Who could have seen that coming?
Unfortunately ... it is probably just the beginning of a lot more "lone wolf" type of domestic jihad-massacres. You don't have to go to Syria, Iraq, or Pakistan to get "radicalized" ... just attend the "right mosque" or frequent the "right web-sites" ... that's all it takes.
Tick ... Tick ... Tick ... Bang - Boom.
Law enforcement / intelligence basically have to be right 100% of the time ... these cockroaches just need to get lucky once.