Its very likely a raccoon. They have hands, and if the chicks were trying to escape and he was having issues with the rooster, his efforts to hold on the thie chicks could have resulted in their being squeezed. Also, foxes and other predators are attracted by the sounds of distress. It could be that it was a double attack, one from a coon, then a scavenger may have taken a few others. Your rooster may have been taken by the coon, they are fierce predators and have been known to kill dogs twice their size. If anyone ever finds a live coon in the coop, they should never ever ever confront it. If cornered, they can become very dangerous. Same with opossums.
When I started having chickens, I read this site up and down, back and forth, all about predators. I have had no attacks and I live with hawks, weasels, fishers, coons, coyotes, foxes, and even had an eagle land on the top of the coop when i first built it. My coop has 2 types of runs, one for when Im gone, which is heavily wired, and one that is less so, for when i am home and in the yard. But despite my precautions, I am very aware that at any point a snake, coon, rat or any other predator could easily get in and cause all kinds of damage. Wire only is a mild deterrent. If something really wants in, its going to get in. However, the more astute you are in your precautions, the less likely this will occur again.
My poor girls never free range unless Im around to watch them constantly. I keep a very close eye on them and walk with them when they are outside the coop, but if Im not around, they go back into the coop.
Im very sorry for your loss. Whatever did this needs to be trapped before it gets the idea that your yard is a buffet. Call your local Taxidermist, they can give you the names of trappers, or call Fish and Game and let them know you have a problem animal. Perhaps they can help you identify, capture and destroy the culprit.