On the thug's body, you find a couple of things that might be clues. Besides the thug's knife, a coinpurse, and some food, you also find a worn red cloth and a piece of what looks like propaganda. The bits of the propaganda that are legible seem to promote freedom and "rebirth".
Turn these in to the people by the wagon. Maybe they'll know something about these objects. Look around the scene. There might be some clues nearby.