It's sure no joke.
But, the media and the people have been speculating about this specific threat for days. The only thing that Moose's statement does is change the statement from one of speculation, to one expressly made.
What was the source of the threat some time ago? TYhe one that implicated threat to students of all ages? For some reason, today's threat to "kill your children" is a minor deja vu for me.