Foxes keep attacking our cats!!!?

I was in my livingroom watchin tv kind of dozing off, this was around 11 at night and i heard a loud squeal coming form outside, it was only 30 seconds later of the squealing did i realize something was getting attacked, so i looked out the window to find a fox just moving its head up and down (it was quite dark and hard to see) and it saw me, looked at me for 5 seconds and trolled off. i dont know if it hada something in its mouth but i went outside to see if anything was out there and i didnt see anything. can someone suggest what that fox might have done? i feel terrible that i could have prevented what ever happened.