I'm sorry that happened to you and your flock. Unfortunately, that's the risk we run when we free range. It could be anything at any time. If there's something in the area that's hungry, and chickens are on their menu, then it's all a matter of timing and luck.
If you didn't have a roo, but are allowed one in your area, you might consider tracking one down and adding it to your flock. A majority do a darn fine job of sounding the alarm when they spot a potential threat, especially an aerial one. Giving the hens a chance to run for cover. Some roos even manage to take down attacking raptors.
Additionally, it'll help if you only free range when you (or someone) can be outside and nearby. We never let ours out unless we're out (which we often are).