So for the question, take these things into account.
Up to very recently, the world was exactly as ours. Everyone from angsty teenagers to modern pagans, fraternities to secret societies perform rituals, for tradition, entertainment, and worship.
A few- a small few- of those books were legitimate and functional, but there was no magic for them to draw in so there was no difference.
Suddenly, a supernatural creature pops up. There are a couple witnesses, but the critter is killed and burned thanks to some quick thinking by average people.
Now, people are developing powers. (Psychics, armored skin, magic use). People get them suddenly, but they're exceptionally rare. Spells from the rare forementioned books suddenly work, reliably.
A low-ranking FBI agent observes something undeniably supernatural.
A body is found, with human DNA. Unfortunately, tests also show inconsistencies like missing organs, or organs/parts that humans dont have, like gills, wings, venom glands, fangs.
Humans are found, that seem completely normal- except they have no identification. No history. Nothing verifiable whatsoever.
So far, nothing very the top has happened. No Cthulhu, no fireballs thrown on live TV, etc. Its the beginning.
How do you think the governments would react? How would they treat people with no identity? What about people who seem to be at a disproportionate number of supernatural events, even if witnesses say they were helping people?
Edit: I mean, very very early on. Covert agencies and imprisonment of mages are in the cards, but this is early. Its rumours. First hand-accounts. No evidence yet. Do they just disregard the current laws to contain it, or would they wait for proof? Twist the current laws? For an analogy we're all familiar with, this isn't a Zombie Apocalypse. This is a single zombie, in a vast national forest. A small-scale reaction. The FBI knows something... weird is going on, but no laws have been broken yet. Perhaps they'd hold mages for unrelated crimes and falsify evidence?
I'll also add... This story is the foundation for an RPG. The people getting powers are receiving them as payment for completing missions. Being players, the aforementioned condition that non PC witnesses end up dead is true. However, the players have so far ratted out one player as having powers (a psychic effect, reaching out to a target and extending a purple flame to engulf them which results in incapacitating pain for a few moments.) The other players now have a power, one can throw fireballs (sigh) and the other can mentally connect with wireless networks. So as of now, there are five people in the whole world with powers. 2 are staying silent (and have no ID, no history) and three are from this world, and will probably end up flaunting their gifts in time. One from this world is an FBI hacker. How do you think police/fbi would react to the accusation of powers? What about the 'proof' of them?