I recently put an end to a night time predator that killed 4 of my chickens over a week's span.
I used a baby monitor placed just outside the coop/run area,powered on a very long extension cord. I ran it night and day, keeping the receiver in the house. It picks up quite a bit of activity, and grabs sound over a fairly wide area. At 4 in the morning, I was woken up by the loud ruckus of an attack, and was able to run out and kill the offending critter while it was trying to corner two hens on the roost.
Alternatively, look into trapping. That's worked for me in the past.
Trapping requires more prep work and knowledge to avoid injury to local wandering pets, but less attentiveness than the baby monitor solution. Check your traps daily, and confirm your local laws on the subject of course.