If this is true, then how can one dispute any conspiracy theory? If one believes that all of law enforcement and intelligence is in on every conspiracy, will never leak, and that all evidence of every conspiracy is destroyed before it can ever be discovered, then you can justify any belief.

I think if the FBI or police covered up a conspiracy theory of planted child pornography evidence, and this were found out, it would be a massive scandal that would dominate the news for months or years.