Though accidents are included in "unnatural deaths".
Something happened to her, but we do not know if another person caused her death intentionally or not. I do hope it can be proven conclusively one way or another.
I don't agree that an adult would "never" cover up an accidental death, partly because so many missing children have never been found, so we can't know the true circumstances of their deaths (Lisa, Haleigh, etc). I am sure at the very least that negligence caused most of these deaths, including Mariah, and perhaps worse but just wonder if LE will be able to prove it. Jmo