Like they said, not a raptor, they tear pieces out. Dang six foot wing spans, doesn't sound like a vulture, more like a condor, or one of the prehistoric birds of Indian lore. There was a story on one of those, on Monsters and legends, some time back. A couple of people said they had seen them, and one had said one tried to get him when he was young boy. and it was all up north your way. You just never know......
Sasquatch?
A coyote could have bitten it in half, they do have sharp incisors behind the canines, and they sometimes will grab a rabbit or other animal, and chomp and shake their heads to the sides like a gator to tear it in half. Or you could have neighbors that could have slung in your yard, or like you suggested, the mower. Lots of possibilities, but I like el chupracabra. Probably was let off by aliens, when it started to eat the other specimens they had acquired.
Put up a trail cam on your lawn.
Sportsmans guide has them on sale.
Wayne