A few seconds after 12:30pm on 22 November 1963, Lyndon B Johnson lay face down on the floor of his limousine under a secret service agent called Rufus Youngblood. Radios crackled with panic and confusion. Neither man knew it yet but the moment a bullet had killed JFK two cars ahead, Johnson had automatically become President of the United States.

The US constitution requires that a new President must swear an oath of office before executing it, saying: "I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my ability preserve, protect and defend the Constitution of the United States".

Fifty minutes after the shooting, at Parkland Hospital, Johnson learnt that Kennedy was dead. Amid fears the assassination was part of a broader conspiracy, Johnson's staff urged him to take refuge on Air Force One, which was parked three miles away at Love Field Airport. Johnson insisted on waiting there for the arrival of Kennedy's coffin and the First Lady before taking off.

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Mindful of the need to be sworn in quickly as a symbolic act of strength and continuity, Johnson removed himself to Kennedy's cabin. Seeking legal advice before proceeding, he called the Attorney General, who happened to be a grieving Robert F Kennedy. The timing, purpose and questionable sensitivity of the call contributed to an already bitter political rivalry between the two men.

Furnished with the wording of the oath and the impression that an immediate affirmation were required, Johnson called for his friend Sarah Hughes, a federal judge in Dallas. The Kennedys also arrived, Jackie's pink suit stained with her husband's blood. With three news reporters summoned, the swearing could begin just two hours and eight minutes after the shooting. Photographer Cecil Stoughton had been picked up by the Johnson people at Parkland Hospital to record scenes on the plane.

Larry O'Brien, one of Kennedy's inner circle, found on a side table in the dead President's cabin a book bound in calfskin and embossed with a crucifix. He took it to be a Bible. In Air Force One's crowded stateroom, where a shell-shocked Mrs Kennedy stood beside Johnson, O'Brien handed the book to Judge Hughes. The oath sworn, President Johnson said: "Now let's get airborne".

In a 1968 interview recorded for Johnson's Presidential Library, Hughes revealed that she accidentally added "So help me God" to the end of the oath she read on the plane. "Every oath of office that I had ever given ended up with 'So help me God!' so it was just automatic that I said [it]."

The book turned out to not be a Bible but a Catholic missal, a liturgical guide to celebrating mass presumed to have been a gift to Kennedy. Its later whereabouts became a mystery. Some suggested Johnson's White House sought to suppress it after its true nature had become clear. It later resurfaced and is now on display at the Lyndon Baines Johnson Library and Museum in Austin, Texas