Not so certain
That conclusion seems undersupported to me.
This could have EASILY been a test run by some organization with patience. Not every terrorist group wants to be in the headlines right now, and certainly state actors don't.
With no suspects, we simply cannot know. What seems really, REALLY unlikely is that we are dealing with a couple of kids or people on a lark. Such people don't have the discipline to keep their mouths shut.
I'm not saying that closing the case is wrong, just that it should be worrisome.