Did it look like a humped back cat with a rolling gait? Racoon.
If it was sleek looking, it was probably something else.
How old were the chickens? How many went missing?
Definitely sounds like you need to stop at the hardware store on the way home and pick up some 1/2" fence for the top...
From the size and the way it killed, it sounds like a raccoon.
You will regret putting the chickens close to your window. Poop stinks. It will also draw rodents to your house, which will draw snakes.
Beef up security. 1/4" hardware cloth prevents paws from entering, and 1/2" to 1" wire...