I remember watching a recent History or Discovery Channel show on the John F. Kennedy assassination. They took the media from that day, the press coverage and the news anchors and even just bystanders taking video and photos of the entire day, and edited it together to form a cohesive narrative of November 22, 1963, in Dallas. It was absolutely fascinating. In many ways JFK was the first president elected due to television's burgeoning role in politics - and television is what so many people looked to after his assassination for answers.
It was media that played the primary role in establishing concerns about a conspiracy behind the killing. The Dallas police force was unused to so much media attention, and the chaos and publicity of the event led to confused eyewitness reports. It makes sense that people would consider a conspiracy. John Wilkes Booth was part of a group of conspirators, although Garfield and McKinley were likewise the victims of lone gunmen. With politics being what they were in the 1960s - people worried about Communists, minority races, homosexuals and mob hitmen - and JFK being who he was, it seems almost impossible to believe one man could have acted on his own.
It's as William Manchester is quoted in the book, and in the television special I saw: "If you put six million dead Jews on one side of a scale and on the other side put the Nazi regime [...] you have a rough balance: greatest crime, greatest criminals. But if you put the murdered President of the United States on one side of a scale and that wretched waif Oswald on the other side, it doesn't balance. You want to add something weightier to Oswald. It would invest the President's death with meaning, endowing him with martyrdom. He would have died for something."
Unfortunately, there is no conspiracy. Gerald Posner masterfully demonstrates this in his book. "Case Closed," which is rife with footnotes and citations, follows Lee Harvey Oswald and Jack Ruby from birth to death, accounting for what seems like almost every day of their adult lives. It is easy to see after reading in so much detail about Oswald why he seized the opportunity to kill the President, and it's likewise easy to see how much emotional turmoil Jack Ruby went through before spontaneously deciding to kill Oswald. Not only that, but it becomes obvious that these two were bit players, maybe not even that. They both had fantasies that they were more than they really were. Oswald wanted to be a great Marxist or Communist. He wanted to be recognized. Jack wanted to be known, as well.
Beyond detailing their lives up to the assassination/murder, Posner also delves into some of the concerns people have with the consistency of evidence in the assassination. The bullets, the trajectory, the number of shooters, the evidence in the Book Depository, and witness accounts are all addressed. This book came out in 1993, so it is somewhat dated, but he was able to use computer modeling to recreate some of the questionable situations. He also uses medical evidence to account for certain things such as why JFK looks like he is reacting to a shot from the front when he was shot from behind.
This is really an excellent book and a must-read for anyone interested not only in the assassination itself but also the history of our country in the 1950s and 60s and the prevailing mindset of the time.