I'm pretty sure you can use speak with dead on Rugan, and maybe with the other guy that was with him too. Those two were the ones using fire to keep the gnolls from entering the cave, but if they died to fire or some other effects, their corpses are too damaged and you won't be able to speak with them.
In case it was you who killed them and now they refuse to speak with you, use the disguise self spell.
