What killed my rabbits, i think its a fox but theres no trails or no signs please help ;D ? xx?

My rabbits have a big play pen with hutches inside, i left them in the pen all night. the next morning i went and then was all gone, they is no way for them to get out at all. I thought it was a fox that ate them but there was no signs off blood or paw prints.. What ate my gorgeous rabbit :'( ?