I feel like this question calls for an attempt at a factual answer. Please understand that I don’t mean to in any way to detract from the extremely justified outpouring of rage against Nassar and anyone who enabled him.
I’m not a lawyer, but, so far as I know: In the United States it is not, in general, against the law to fail to report a crime. People in certain professions are “mandated reporters” when it comes to certain crimes – e.g., a social worker, pediatrician, or school teacher may have a legal duty to report suspected child abuse. [That might apply in the Nassar case, given the nature of the crimes and the ages of the victims, but I think it would depend on who it was who had reason to suspect what was happening.] But where that sort of requirement doesn’t apply, I don’t believe you can be prosecuted for failure to report a crime. On the other hand if you help a suspect escape, or hide evidence, or lie to the police, you might be charged as an accessory to the crime that was committed (“accessory after the fact”), or with obstruction of justice. If you lie in court, you could be charged with perjury. If you refuse to answer questions when ordered to do so by a judge, you could be found in contempt of court (assuming you yourself are not on-trial, and that you aren’t someone like a spouse, lawyer, or clergy for the accused, to whom “privilege” may apply). When questioned by the police, you have a right to remain silent if you’re detained or arrested (with a few exceptions – for instance, some jurisdictions require you to give your name and address if asked), and if not detained you can simply leave. Again, I’m not a lawyer, so take this with a large grain of salt.
I should emphasize that whether or not you have a legal obligation to report a crime is a different question from whether you have a moral obligation. I would never blame the victims of sexual abuse for being afraid to report it, but for everyone else, the phrase “there’s a special place in hell” comes to mind. Moreover, even when no crime was committed, other consequences may be appropriate, such as termination of employment and the opprobrium of the public.
Also, what the law is isn’t always the same as what it should be. I’m somewhat inclined to think that for certain crimes – especially acts of violence committed against minors – that everyone ought to be treated as a mandated reporter. (I say “somewhat inclined” because I hadn’t really considered the question until now, maybe I’ll start a GD thread on it.)