Ok.. I understand that it must have been the escape, but what I don't understand is what they might have brought back?

The room they are in is unused and empty. There are a couple of things down there and nothing was chewed or anything, and the room was completely cleaned and emptied.

I'm trying to figure it out more than just 'they escaped, they died' because I can't imagine something that could be brought back to kill all of them in less than 24 hours. If it was food, what could it have been? What could kill all of those rats?

I've been running through lists of possibilities, and I've dealt with various animal deaths of every variety.. this one caught me off guard. :/ lesson learned, but the lesson still hasn't explained the 'why'

Options are:

food
poison
chemical
picked up disease?
feces (I had a cat down there, which is the reason why everything was so hardcore cleaned once the cat left)
so, maybe litter?