Seems like a fake bomb threat / provocation to me, but difficult to see motive. Laughed about the explanation why not done by kids 😂
First, because they think (mostly) about themselves
Why fake?
First, because fake is always easier than doing the real thing. Second, attacking so many buildings at the same time requires skills, logistics and a bigger organization that could probably (hopefully) be tracked down. Third, motive letter would be clearer why it's done if it was real.