When doing that, I follow what I saw done in LARP.
Pre-gen character, making sure that everyone has a secret, a couple of goals, and relationship with character. Then I find an "in-game" reason to keep everyone in the same-place (snow storm, hiding from the police, magical myst, on a boat), Finally, I had a couple of clues which may reveal some secrets over the game-location, and sometimes a couple of "events", but usually letting "brew" is enough to have the plot revealing and the betrayal occuring. As any game with some PvP component, keep some time for debrief/cool-down, it can be emotionally intense
As opposite to LARP, I would still have a bunch of NPC, who would also have their goal/secrets. However, I would try to keep the "big secrets" and "culprit" within the PC.
IMO it's harder to incorporate this kind of things in an exiting campaign, or you go to a more classic investigation (which are nice too),
Article de FranceTV
https://www.francetvinfo.fr/economie/medias/wikipedia-publie-une-lettre-ouverte-pour-denoncer-les-menaces-d-un-journaliste-du-point_7081668.html
Ça sent le bel effet Streisand pour _ Le point_