It's taken me a while to realize that no one in this discussion knows that you can't libel the dead. It's true. You can't. Libel laws apply only to the living.
If the dead criminals you want to write about have any living relatives, and you've said that they were participants or otherwise complicit in the crimes, they might have a case. If you didn't say that, simply being related doesn't give them standing.
You've said the police committed various misdeeds as well. If you've identified officers by name as being among the guilty, they might have a case.
Anything that's a matter of public record is fair game.
If you can prove something is true, it isn't libel.
If you're presenting your work as nonfiction, learn to be precise about your sources. If so-and-so said something useful and colorful for which there's no firm evidence, "So-and-so said [whatever]" is okay, because what you're asserting is that he said it. However, if your omniscient narrative voice simply repeats whatever that thing was that he said, then you're asserting the thing in its own right, and "So-and-so said it" is not a defense.
Allen Brady is mistaken on a number of points:
A novel would be more problematic than a non-fiction work.
No. It has a different set of potential problems. Given what you want to do, a novel might be easier to manage.
In most jurisdictions in the US,
In
California plus twelve other states.nineteen states now, but those laws are seriously inconsistent from state to state.
your heirs control your personality rights for 75 years after your death.
That only applies if you're a bona fide celebrity, which for purposes of this law mostly means that your "name, voice, signature, photograph or likeness" has commercial value. If Bill Cosby died tomorrow, Royal pudding couldn't hire a Bill Cosby impersonator to do knockoffs of Cosby's Jello pudding ads.
How "personality rights" interact with freedom of speech, the right to say things that are demonstrably true, the interests of historical analysis, and other speech rights, has not yet been sorted out. What this means in practice is that whoever has the biggest lawyers gets to play the bully. Thus, you could in theory publish pornography starring Franklin and Eleanor Roosevelt, Richard Feynman, Omar Bradley, or Rosa Parks, but the likeness of Sonny Bono is sacrosanct until 2073.
(Rant: The very existence of that law is a gross distortion of the legal system. Why should the heirs of celebrities have rights no one else does -- continuing for
75 years after their deaths? It's just one more bought-and-paid-for Hollywood law for which there's no sane justification: a standing affront to the public good and the common discourse.) (/End of rant.)
So you're free to write a book about Abraham Lincoln hunting vampires, but if you want to write the same book about Ronald Reagan, you would have to get permission from his heirs.
If it's parodic, satirical, or otherwise functions as commentary, you're on firmer ground. To use a different strategy, if what you say is demonstrably true, you're also on solid ground.
Practically speaking, the more literary oomph you can muster, the more rights you have. This isn't fair, and strictly speaking it isn't the law, but it's how things work. If your book has literary pretensions, cover quotes from distinguished sources, and it's published by the likes of Viking, or Atlantic, or Farrar Straus Giroux, you can get away with things no trashy paperback could ever dream of.
The same restrictions don't apply to non-fiction.
Again, a different set of restrictions apply.