Who does Sheriff Tate think killed Bob Ewell in To Kill a Mockingbird?

At first he tells Atticus that Bob fell on his knife. But he knows that Bob was murdered. He also knows that it wasn't Jem, which Atticus doesn't know. But the fact that Boo doesn't want to known and followed by paparazzi, which may happen if people know he saved two kids lives, the Sheriff tells Atticus that Bob fell on his knife.