Predator in the barn, what could it be and what to do?

I have three separate coop areas, a 10 x 12 raised hen house that was built on poles like a pole barn, completely enclosed and shut up at night, a stall in a lean to shed attached to the barn, and a 12 x 12 stall in the horse barn. After finding a skunk in the stall in the lean to shed one night I invested in NiteGards and have had no problems since....UNTIL....drumroll here........last night. The 12 x 12 stall in the barn is where i raise baby chicks I hatch and purchase...there are probably 400 little chicks in there and I move them out as they get bigger....Last night, something dug under the stall wall from an adjoining stall. I had no dead chicks out in the middle, but there were 20 dead chicks behind two oak doors that I had laid on their side and leaned up against the wall. The chicks behind door number 1 were mostly just dead, three were missing heads and one still had its head but was missing it's intestines.
I am perplexed....there were some scattered feathers near the doors but now what to do? I close the barn doors at night, but there is room for a small animal to go under the doors....and now that whatever it is knows there is helpless lunch in there...I am unsure what to do....I did not have any of the nite gards on the barn doors.
I could
A. Leave my dog shut in the barn tonight and hope it is not a skunk so I dont have to do the skunk smell bath tomorrow..
B. Move all the chicks to the 10 x 12 hen house...where they would be overcrowded because there are laying hens in there but they would surely be safe...
C. Sit out there tonight with the pistol and hope I don't miss when it comes back.
D. Surround the stall with an electric fence. (Do electric fences deter determined hungry predators?
E. I do have live traps and am thinking perhaps set a trap with some eggs and some sacrifical chicks in it to try and catch it.
I am open to any suggestions for any other solutions. I don't have a husband or boyfriend to help and dont know if I can get the handyman to come by today.

I know I have skunks, and my daughter had a troublesome raccoon last year but she lives a mile away. My neighbor said he saw a fox last week. I live in the middle of the prairie, there are no trees for cover for over half a mile so I have had little problems before. I now realize that I should not have this many chicks and will advertise to try to sell them rather then lose them...but the cabinet incubator had great success and it was addicting....(Oh, I will hatch just a few more eggs).

Thank you for any and all help.

One other thought, I do have hog panels, poultry netting and hardware cloth....does anyone think that putting any of those down around the inside of the stall or on the outside of the stall wall would keep the predators out? The stall walls are solid rough cut lumber 2 inches thick and the side walls go up six feet, the front wall goes up four feet and then has stall bars that go up another two feet so unless the predator can climb a six foot wall, it has to go under, not over....

I find that the raccoons go on a rampage and behead them and leave the bodies....

That is what is so odd, there were 20 dead chicks, all behind the doors leaning against the walls, two were missing heads, two more missing guts, the rest were just dead....of course, there were some scattered feathers so perhaps the midnite snacker ate some in their entirety.

Weird. I lost some chicks a few years ago and found the remains in tight spots underneath stuff. Turned out to be rats. I dunno. Hope you figure it out soon!

maybe put an apron of harware cloth along the outside of the stall running half up the wall & half as an apron on the ground & pin it down or weight it down so that the predator has a very very long way to dig and can't chew through the stall wood. or if you have some flashing around, use it like edging on the inside of the stall so the critter runs into a metal wall when digging from the outside.
it kinda sounds like something fairly small.

Is anyone else a little disturbed that you would use some chicks as bait?? Lock them up and get a game cam.

^---yeah, that.

I think (if I'm understanding you right) that adding 400 chicks to a hen house would be incredibly stressful to the hens and dangerous for the chicks. You'll certainly lose some.