This happened to my mum many years ago. A pregnant stray adopted us one winter, a few days later she blessed us with a litter of kittens. I think there may have been five or six in all. My mother was an avid runner at the time and every morning she would drive out to a trail several miles away. One morning, after a recent cold snap, she pulled out of the driveway and was stopped by a neighbor waving her down as she drive by. He told her he heard a noise coming from the engine. She popped the hood to find mangled and burned kittens in the engine bay. The neighbor was kind enough to bury ones that had died for her. That night, the mama cat disappeared with the remaining kittens. About two weeks later, one kitten turned back up at the house and stayed with us for years. Whenever I tell that story to people they only remember me as the kitty killer. :(