Nicholas Katzenbach: "The Katzenbach Memo", Mary Ferrell Foundation.
On the day of JFK's funeral (25 November 1963), Deputy Attorney General Nicholas Katzenbach sent a memo to Bill Moyers (then presidential assistant to Lyndon B. Johnson), calling for a campaign of action to satisfy the public that Oswald was the lone assassin, and that the evidence for this conclusion was compelling, well before the truth of the matter could possibly have been known.
"The public must be satisfied that Oswald was the assassin; that he did not have confederates who are still at large; and that evidence was such that he would have been convicted at trial."
"We need something to head off public speculation or Congressional hearings of the wrong sort."
Links to images of the memo
Image: "Nicholas Katzenbach at White House, 6 May 1968.jpg", [public domain] via Wikimedia Commons