No overt signs of interrogation, nope. All personal effects have been removed from their persons save their clothing. They've all been killed execution-style by a single shot to head, and although the trajectory is consistent with a bullet (more specifically, a musket-shot style bullet, rather than a conventional bullet) the wounds appear slightly cauterized, suggesting an energy component to the weapon.

EDIT: For reference, this will only be discoverable if the bodies are taken and examined closely, it cannot be discerned from distance observation.