They're looking for crazies that go on shooting sprees. What they should be doing is checking EVERYONE... not just people that publish books about school shootings while working at schools.
I frankly don't have a problem with them investigating the guy so long as they do it respectfully. That said, everyone should be checked out. These mass shootings are just crazy people acting out. Nothing more.
OR, and I know this is a long shot, but they could consider accepting that a free nation is an inherently dangerous one, and either accept that individuals have a right to live their lives without constantly being spied upon by a crazy government so paranoid it makes tweekers seem like reasonable people, or find some other, "safer" country to move to.
I know - leaving other people to their business so long as it's not directly and immediately affecting you - crazy idea, right?