I know of someone who did that with one workplace. Massive taxpayer fraud going on, millions taken from public funding and redirected to business bank accounts. Evidence provided included where and when to look, amounts stolen, that kind of thing.

What they didn't know at the time was the person who was signing the checks was an ex-employee who had moved up the rung into regulatory, and was responsible for receiving the letter. No investigation, in fact they got a big increase in the level of funding. The fraud still goes on.

Nobody there knows who sent the letter, because if they did the author would have received a visit from some associates and probably be in a care facility somewhere, or possibly even dead.