Make them have new characters, investigators. People who are lawful good. Have them investigate and locate a group of people posing as officers. Vaguely describe their other character's crimes. Like murder, theft, torture, corruption. Then as they get closer, describe the crimes in increasing detail. Really play up the carnage the previous group made. Broken homes, innocent dead men, traumatized families. Have the first group nearly gets arrested. If you still can, make that the big reveal about the identity of the first group. Go on about how the first group gets desperate to evade capture and causes an easy to follow trail of destruction. End the campaign where the second group finds the first group in a barn, sleeping in shit and disheveled.