As requested over in the
Troublemaking criminals thread, savegame with all three combat mod options off (Size Matters, Fight or Flight, Hide and Seek) showing inability to get rid of spawned criminals in my cities. SVN9199, I believe.
Criminals in question are a mixed bag parked in my capital of Bibracte, on the west coast of my main continent. There are more criminals in the cities in the far north and along the east coast of the continent. None of the criminals in any cities have accumulated any Wanted promotions, which is likely the reason for inability to Arrest. I believe what's happening is that the Investigation check is succeeding once in a while (granting XP to investigating units), but it's failing to add the Wanted promotion to the criminals.
Assassination attempts against them also fail, silently. In the capital, there are a Game Hunter, Assassin, Hashishin, and a pair of Scoundrels. The first three do not have Criminals on their target list, so should not be able to assassinate them if I'm understanding the mechanic correctly. The last does, and should. Mouseover of the Assassinate button gives no target or odds for the first three units. Mouseover on the Scoundrels' Assassinate buttons DO give odds against one of the Troublemaking Criminal Assassins, but not all of the time. In all cases, clicking the button results in a popup with options of "______" (blank) and "No", neither of which has any effect.
Finally, if any of the assassination-capable units leaves the city and attempts to reenter, it will be blocked from doing so. Sometimes, a brief flash of combat odds occurs if the unit in question is a Scoundrel.
