The Katzenbach memo is proof that there was no good faith effort by the government to rule out a conspiracy. Lawyers don't ask questions that they don't want to know the answer to and politically, the lone assassin narrative is better than trying to explain a conspiracy to the public (whether it was an inside job or done by foreign enemies it would be a political minefield).
The memo didn't mean that Katzenbach believed there was a conspiracy that had to be covered up. It meant that many--going by the first-day evidence and no evidence of other suspects appearing over the weekend--genuinely believed by late Sunday that Oswald was the lone assassin.
In fact, if you dig down into the memo, Katzenbach is concerned about the Dallas police and Texas rightwingers exploiting the assassination to wage a McCarthyist witch-hunt.
"Speculation about Oswald’s motivation ought to be cut off, and
we should have some basis for rebutting thought that this was a
Communist conspiracy or (as the Iron Curtain press is saying) a
right–wing conspiracy to blame it on the Communists. Unfortunately
the facts on Oswald seem about too pat — too obvious (Marxist,
Cuba, Russian wife, etc.). The Dallas police have put out statements
on the Communist conspiracy theory, and it was they who were in
charge when he was shot and thus silenced."
It also shows that Katzenbach thought the rumors were getting out of hand in the wake of Oswald's slaying by Jack Ruby. And that Katzernbach thought "the facts on Oswald seem about too pat".
These were concerns Katzenbach didn't want swept under a rug but presented in a public report ("a statement that all the facts will be made public property in an orderly and responsible way should be made now."), with the hope that the FBI report might suffice. If not:
"The only other step would be the appointment of a Presidential Commission
of unimpeachable personnel to review and examine the evidence and
announce its conclusions."
Katzenbach didn't want the showboating or potential Mccarthyism of a Congressional hearing:
"We need something to head off public speculation
or Congressional hearings of the wrong sort."
The FBI put together a memo shortly after the assassination concluding that none of the shots missed the limo.
As already mentioned, the agents at the autopsy reported initial speculation at Bethesda which was later rejected by the pathologists and every subsequent medical review board. By Sunday, the autopsy report had concluded there had been transit of the neck.
Then the Warren Commission followed the evidence that led them to the ingenious Single Bullet Theory (or Fact). Critics are jealous because they have nothing as clever (yet Occam's razoresque) and scientifically-sound that "solves" the assassination. Their lamebrain theories and goofball trajectory gyrations will always come up short.
The injuries suffered by James Teague forced the Warren Commission to consider a theory of how just two bullets caused all the wounds to Kennedy and Connally...
No, it was more what happened to the near full-velocity bullet that left Kennedy's throat if it didn't damage the limousine. What was forward and slightly to the left of Kennedy that wasn't a part of the vehicle? The Governor.
