WASHINGTON — Who killed JFK? Fifty years after the slaying of the nation’s 35th president, that’s still a provocative question for many. Conspiracy theories began swirling almost immediately after President John F. Kennedy’s assassination in Dallas on Nov. 22, 1963, and have never really stopped. A spate of new books re-examining that moment in anticipation of the 50th anniversary has revived some theories, tried to squelch others and found intriguing new details of botched investigations or deliberate concealment by authorities.