Some of you guys have an inordinate capacity for suspension of disbelief. I like the DV as much as anyone, but good grief. A plot hole is a plot hole.
You guys are attributing the US government with plot power comparable to ministry of magic in Harry Potter. And the other half of you is giving human rationalization totally unrealistic capabilities. It doesn't work this way IRL.
Note I'm not saying the cat is totally of the bag here. But people would absolutely, totally, completely certainly, without a doubt, indubitably, know that something strange happened. They may not know what kind of animals attacked them. They may not even think it was magic. But its crazy to say that the general facts of the incident would not be known. Not. Possible.
Oh, they'd be known. But so would 200 other 'facts' about the incident that didn't happen to be true.
You're right that trying to pretend that 'nothing happened' would be futile. That's not how you'd go about it. Instead, you'd cloud the issue. You get a starting help from the chaos and confusion that accompanies any nasty incident like that, on any scale.
On a larger scale, it would be like a mass shooting. Think about the confusion that usually accompanies such incidents. There's 1 shooter, no, 3 shooters. There are 15 dead, no, wait, 12 dead. The shooter was armed with an AK-47. No, wait, it was a pair of Glocks. No, wait, it was 2 shooters, and they were shooting at each other.
It usually gets sorted out over a day or two, but imagine if somebody in a position of authority was deliberately trying to cloud the issue and confuse the matter. Now imagine that it involves lots and lots of confused, scared, and mixed up witnesses, with conflicting accounts. Imagine that somebody has his own people claiming to have see 'x, y, and z' and x, y, and z are all made up at the same time. Imagine that those same someones are deliberately clouding the issue every step of the way throughout the investigation.
The forensics found weird stuff? Put out a story that they also found imaginary weird stuff B, C, and D, and then have somebody else 'debunk' B. Use the protests of the real witnesses that C and D didn't happen as yet more fodder for the confusion.
You've got a cop or an FBI guy who won't take a hint and shut up? Remind him that he's still 10 years from retirement, has a kid in college and mortgage, and jobs are hard to come by. If you're really ruthless, he can have an accident.
If somebody has the necessary money and connections, they could really confuse the issue to the point that nobody, even among the authorities, was sure of anything. Then you put out your sane-seeming, rational-seeming explanation. If necessary, fake up a reason to divert a bunch of FBI or police resources elsewhere for a while.
It's the same thing Murphy used to do as head of S.I., on a grander scale and with a bigger budget.
It's not infallible, of course. But it's not as hard as all that if you've got the resources.