How horrible!! Did you find an entry point? Go over your set-up with a fine tooth comb - checking for any weakness, or places wires were bent out of shape , broken or gnawed on. Do you see any feathers not belonging to your chicks? Any fur or hairs (dog, cat, wild preds?) Under holes or tunnels around the outside of your pen or up thru the floor - look under their little box etc. rats would do that. Are there any paw prints, or signs of a scuffle?
What a heartbreaker but, could you get a game camera or borrow one? Whatever came was successful and will be back? Maybe you could leave parts of a store bought chicken inside the coop and wait up for it(them) to return or - set a camera aimed at your site to see what turns up.
If you discover anything amiss please let us know.I am subscribing to this thread because I am hoping to hear you found the pred and dealt with it. Life teaches us some very hard lessons but, if you solve this one - you will find out how to prevent it from happening again. I am so sorry for you and the babies.