I had several losses recently from rats. The rats attacked the chickens by biting them, then removed their heads. It took some work to get rid of the rats, which were apparently attracted by the big farm totes of feed, but I believe we finally won the battle. For now. Haven't seen a sign of one for about 2 months. The difference is they only got in the coop when the automatic pop door opened one morning and I interrupted their attack when I went to check on them. The day was the last day of losses. Two were beheaded outside, and two were killed in the coop but not yet beheaded. There were two other days: the first was one hen (of course my favorite) who I found laying on the ground. When I went to see what the heck was going on with her, her head was gone and her neck, eaten down to the bone, was down what I quickly learned was a rat hole behind the coop, outside of the run. That was very sad. The next attack was a day or two later when I lost two more by beheading by their feeders in the run. Then the final attack as described above. My birds free range, but they have a very secure run and absolutely secure coop when the pop door is closed. After the third attack (because I was having a hard time grasping that I actually had a problem, not just some mishaps), I lock the birds in the run and waged war. We ended up killing many rats and I didn't know we had a problem prior to this! I had only ever seen a rat once as a child dead in the street and once at our place in a bench where I stored bags of bird seed shortly after we moved to the boondocks, and long before I ventured into chickens.
I don't know if you have rats, or if its weasels, but my situation is similar. And I feel for you. Besides my favorite hen that was killed (she was the first egg my broody ever hatched and the only one from that clutch), the others were all cornish cross and about three weeks old. It was sad. I feel totally responsible (or irresponsible!). And I hope to never have to deal with this again.
I'm sorry you have to deal with it now.