A 17-year-old from Nebraska and her mother are facing criminal charges including performing an illegal abortion and concealing a dead body after police obtained the pair’s private chat history from Facebook, court documents published by Motherboard show.
Actually that page suggests that they can't access it. They'd never passed the security on it if that page was lying and they don't encrypt it. Clearly there must be some kind of mechanism they can use to decrypt it for law enforcement. The technicals of that are what I was actually interested in from my original comment.
EDIT: Oh my God I just figured it out. It's not enabled by default. You have to explicitly turn it on per conversation. That's terrible