(CBS News) It's been nearly 50 years since the assassination of President John F. Kennedy. Questions still remain about whether Lee Harvey Oswald acted alone or was part of a larger conspiracy.
Now there's new evidence about that fateful day. It comes from a book called "The Kennedy Half-Century," written by professor Larry Sabato, director of the Center for Politics at the University of Virginia.
From the moment shots rang out in Dealey Plaza the search for definitive answers in the Kennedy assassination has proved elusive. Was Oswald acting alone, or was he a member of a conspiracy?
The 888-page Warren Report issued in 1964 found no evidence that anyone assisted Oswald in planning or carrying out the assassination. The report had many critics and conspiracy theories multiplied over the years. Hundreds of books have been published about the case and dozens of documentaries and films, most notably Oliver Stone's 1991 Academy Award-winning film "JFK." But the strongest official confirmation for conspiracy buffs came in 1979 when the House Select Committee on Assassinations concluded that President Kennedy was "probably assassinated as the result of a conspiracy." A key piece of evidence was an audio recording that the committee believed captured the sound of four gunshots being fired. One of the gunshots apparently came from a second location, the so-called Grassy Knoll, a patch of land that was ahead of the president's limousine.
This year, political scientist Larry Sabato had the tapes re-analyzed using state-of-the-art technology. He says they do not capture gunshots at all, but the sounds of an idling motorcycle and the rattling of a microphone.
Sabato also says analysis of the recordings showed the sounds -- which were of police radio transmissions -- were not from Dealey Plaza, but from a location more than two miles away.
A new poll conducted as part of the book found 75 percent of Americans still reject the Warren Commission's conclusion that Oswald acted alone.
Sabato said on "CBS This Morning" his book has completely blown the 1979 House Select Committee on Assassinations report "out of the water." He added, "Their evidence simply does not hold. And they concluded there was a conspiracy. Does it mean that no one encouraged Oswald or that Oswald had no compatriots working with him? I can't say that for sure because the Warren Commission was also deeply flawed. They made so many mistakes in their process, they didn't interview key witnesses that I interviewed 50 years after the assassination and I was stunned to find out they weren't part of that study."
Sabato said he believes Oswald was the gunman who shot both President Kennedy and Gov. Connally. "If anyone else participated beyond the picket fence on the Grassy Knoll, they either missed or didn't fire," he said.
However, Sabato said we're never going to know what really happened. "If we can go 100 years into the future, I guarantee you whatever replaces television, there will be documentaries proposing new theories about the Kennedy assassination," he said.
He continued, "That's because the Warren Commission did not go down the hot trails when we had the chance to interview the right people. They could have done it but essentially they were lied to by the CIA, the president at the time, Lyndon Johnson did not want a thorough study, and so we didn't get one. It was on a political timetable with a political conclusion that Oswald acted alone."
Investigators were under pressure to report and report quickly, Sabato said. "They may have gotten their conclusions right, but it was such a flawed process," he said. "And this is the murder of a young president. This is the youngest president ever elected. The youngest to die. We needed a thorough investigation and the American people would have waited and paid whatever was necessary to get it."
Sabato called both the government investigations -- the Warren Report and the House Select Committee on Assassinations report -- "flawed."
"When you really get into the details, it is amazing how many pieces don't fit. Just to cite one, right after the president was shot, some Dallas policemen ran up the Grassy Knoll and they encountered people who had Secret Service credentials. They let them go. They had their guns drawn. They let them go. You know what they found out since? There were no Secret Service agents in Dealey Plaza, they were all with the motorcade. They went with the motorcade to Parkland. Who were these people?"
Asked about other conspiracy theories, Sabato said he can believe anything, but the problem is "where is the proof?" He said, "In the end, you have to have proof. What could stand up in a courtroom?"
In addition to the theories surrounding his death, Sabato turned to other discussion from his book, including his assertion that Kennedy would not have survived his presidency because of his thin security detail. "John F. Kennedy loved to mix with people," Sabato said. "There would be crowds of hundreds of thousands and they would engulf him with virtually no security. It's amazing he lasted until Nov. 22."
Sabato's "The Kennedy Half-Century" goes on sale Tuesday.