Mission: Investigation & Diplomacy

Even better, have them take NPC’s that are on different sides of each other as a friend/enemy/mentor and then you can most likely lean on their BIG’s quite easily as they will be conflicted with there own patrol mates.
Also, from a GM perspective I would introduce this Mission rather early (maybe the 2nd or 3rd Mission) so that a lot hasn’t been established about the NPC’s otherwise it may throw a wrench in the Mission if one of them has already died or been established as something else that doesn’t fit the Mission.

-LaffinJoker