In chapter 30, what does Mr. Tate say that lets Atticus and Scout know that Boo killed Bob Ewell?

After much arguing, finally the sheriff yells out that he's not trying to protect Jem (he is trying to protect Boo). The sheriff urges Atticus, this once, to accept the situation even if it's not perfect according to law: Mr. Ewell was responsible for Tom's death, and the sheriff urges Atticus to "let the dead bury the dead." He says that it would be a sin to drag shy Boo Radley out into the limelight, and declares officially that Mr. Ewell fell on his own knife. Atticus, deeply moved by this revelation, asks Scout if she understands. Scout assures him that she does, explaining that having it another way would be like shooting a mockingbird.