Woah... Well, first, I am really sorry for your loss. I know how pets can feel like a part of the family. But I gotta say, it really sounds like a person killed your cats.
If they were killed by ANY kind of wild animal, there would be a lot of evidence of it: wounds, chunks of missing fur, and a lot more blood.
It is possible that they were hit by a car and sustained internal injuries and just made it back to the yard to die, but it seems unlikely that they were BOTH hit.
It could be poison, but again, it seems unlikely that BOTH cats would find a source of poisoned food, eat it and die at almost the same time.
Judging by the fur in the yard and the fact that the only trace of blood was coming from one cat's eye, I'd guess that they were killed by someone. They could've been drowned or had their necks broke or been strangled. Keep your other cat indoors. And I recommend alerting your neighbors. If they have pets, they may want to keep them indoors.
I would take them to the vet to determine the cause of death, just for peace of mind. The situation you described is very disturbing and seems unnatural.