Who does Atticus first believe is responsible for Bob Ewell's death in To Kill a Mockingbird?

Initially, Atticus thinks that Jem killed Bob Ewell. He mentally prepares himself for his son having to face a judge in court for the murder and tells the sheriff he hopes the judge will rule Jem killed Bob Ewell in self-defense.  

The sheriff stops Atticus, saying, "Mr. Finch, do you think Jem killed Bob Ewell?" Atticus tells him he does. The sheriff tells Atticus Jem did not do it.

Atticus misunderstands the sheriff. He thinks the sheriff is implying a cover-up of the whole situation. Atticus wants things to be handled properly and does not want people to gossip that Jem got out of it because of his father for years to come. Atticus says covering the situation up will "be a simple denial to Jem of the way [he has] tried to raise him."

The sheriff tells Atticus, "Bob Ewell fell on his knife." He demonstrates what happened to show Atticus. After much arguing, Atticus realizes Boo was the one who stabbed Bob Ewell. To protect Boo, he agrees with the sheriff that Bob Ewell fell and killed himself.