The police, should in such cases, investigate just enough to not look like a bunch of dumbasses, if it would have turned out to be a story to cover up a murder or mansluaghter. Going as far as a grand jury on it was probably done just for that sort of reason, to make sure they did everything right by everyone, but not being allowed to be manipulated by skilled criminals, not that anyone involved wanted to charge a father acting like a parent should for a crime. If it happened with no other witnesses around, it reads like everyone did the right thing, except the guy that died.