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

The normally clear-headed Atticus Finch seems to have disappeared on the night of the attack on his children in the climax of To Kill a Mockingbird. The events of the evening has confused Atticus, and he appears to believe that his son, Jem, has killed Bob Ewell.  

    "Mr. Finch, do you think Jem killed Bob Ewell? Do you think that?"
    "You heard what Scout said, there's no doubt about it... he probably got hold of Ewell's knife somehow in the dark...

The sheriff knows differently.

   "Mis-ter Finch, hold on," said Mr. Tate. "Jem never stabbed Bob Ewell."

The sheriff goes on to explain that with Jem's injury, he could not have had the strength to stab Ewell. But Atticus' mind is not clear--

"I guess the thing to do--good Lord, I'm losing my memory... Jem's not quite thirteen... no, he's already thirteen--I can't remember. Anyway, it'll come before county court--"

Sheriff Tate wonders aloud why Atticus is not already in bed from the shock--but it is also unclear if Atticus has an ulterior motive for his actions. Both Atticus and Sheriff Tate make it obvious that they do not want to have to involve Boo "and his shy ways" in a well-publicized trial. It is possible that Atticus is willing to allow Jem to take responsibility for Ewell's death in order to prevent Boo from having to appear in court. (Whether Jem is a minor--age 12 or 13--would be important if a trial is necessary.)

It seems clear that Boo is the only person strong enough to restrain and then kill Bob Ewell, and perhaps Atticus is now convinced as well. But Sheriff Tate eventually takes control of the situation. He declares that "Bob Ewell fell on his knife"--the blame will not fall on Boo or Jem.

"Let the dead bury the dead this time, Mr. Finch. Let the dead bury the dead."