Sen. John F. Kennedy with Sen. Lyndon B. Johnson, during the Democratic National Convention in Los Angeles, Calif., July 1960. AP It's not just crackpots who question the conventional wisdom that Lee Harvey Oswald acted alone when he killed President John F. Kennedy on Nov. 22, 1963.

University of Virginia professor Larry Sabato, author of " The Kennedy Half-Century: The Presidency, Assassination, and Lasting Legacy of John F. Kennedy" argues that "the chance of some sort of conspiracy involving Oswald is not insubstantial."

Sabato reached this conclusion after considering 50 years of evidence, even while also debunking a conspiracy theory put forth by a House committee in 1979.

"For all attempts to close the case as 'just Oswald,' fair-minded observers continue to be troubled by many aspects of eyewitness testimony and paper trails," he writes.

The founder of the UVA Center for Politics opened this never-ending debate "because the assassination is critical both to understanding America's past and future paths and to the lasting legacy of John Kennedy that is the subject of this book."

Alternative theories cannot be put to rest because of discrepancies and inadequacies in the initial response to the assassination.

To start, there are the questions about why the autopsy was performed at the Bethesda Naval Medical Center in Maryland, not in Texas as required by the law, and why the Bethesda team did not confer with doctors from the Texas trauma room and did not have the president's clothes.

"[The autopsy] opens it up to conspiracy theories immediately that the body was altered, the wounds were altered, and all the rest of it," Sabato told us. "I understand why they couldn't leave the body there but it would have been so much better if it had been performed in Dallas."

More questions arise with the investigation ordered by President Lyndon Johnson, which Sabato claims was haphazard and inadequate.

"The problem is the Warren Commission did not do a thorough job when the trail was hot," Sabato told Business Insider. "The trail went cold decades ago. It is virtually impossible 50 years later to put all of the pieces back together. I've interviewed people 50 years later that the Warren Commission never interviewed that were right there and took important photos or films."

Because of these errors, certain conspiracy theories may never be put to rest.

President John F. Kennedy is seen riding in motorcade approximately one minute before he was shot in Dallas, Tx., on Nov. 22, 1963. In the car riding with Kennedy are Mrs. Jacqueline Kennedy, right, Nellie Connally, left, and her husband, Gov. John Connally of Texas. AP

The conspiracy theories

While all evidence suggests that Oswald killed Kennedy, some clues suggests that he may not have been the only assassin or that he may not have acted alone.

First, there remains "the live possibility of a second gunman in the grassy knoll area," given the testimony of several witnesses, the presence of phony Secret Service agents, and the armed men seen in the vicinity of the Dealey plaza before, during, and after the assassination.

Adding to this theory is Dr. Robert McClelland, a physician in the trauma room of the hospital where JFK was taken, who contends that Sabato notes that the "ambiguous nature of the visual evidence" has led experts to disagree as to whether the bullet that entered JFK's head came from the rear (where Oswald was) or the front (the grassy knoll).

As for the idea that Oswald received help or encouragement, there is no proof that he did, but there also is no proof that he didn't — and there are reasons to be suspicious.

<"For a complete nobody, Oswald certainly did seem to hang out with well-connected people," Sabato told BI.

Some of those shady connections include:

Upon returning from his short defection to the Soviet Union, Oswald became friends with an international man of mystery named George de Mohrenschildt, who "had ties with American intelligence and the State Department ... and killed himself before he could testify to the House Committee on Assassinations," Sabato said.

When Oswald moved from Dallas to New Orleans, he moved in with his uncle, a small time hustler and bookie for New Orleans Mafia boss Carlos Marcello. According to an FBI informant, Oswald received money from one of Marcello's chief lieutenants.

It was in the Big Easy where Oswald became acquainted with the FBI. Oswald handed out pro-Castro literature with the address 544 Camp Street on it. Curiously, FBI agent Guy Banister and a CIA-backed Cuban Revolutionary Council also rented space at the same location.

"One thing that I've always wondered about is [Oswald's] time in New Orleans because he was apparently associated with Guy Banister, who clearly had FBI and CIA ties, and yet he's also scuffling on the street with [the local representative of] an anti-Castro group," Sabato said.

When Oswald was arrested after the fight, he demanded to speak with FBI, and the agency sent Special Agent John Quigley to see him.

"All of these things are suspicious," Sabato told BI.

As a teenage Oswald was photographed with David Ferrie, a staunch anticommunist who would allegedly go on to buy weapons from mob boss Marcello and hand them off to Banister and a CIA asset. Ferrie denied ever knowing Oswald, yet in September 1963, six witnesses alleged that the two showed up at a voter registration office in Clinton, Miss.

"It could be that Oswald was just a Forrest-Gump like character who popped up at interesting moments wherever he happened to live," Sabato writes. "But just as conceivably, whether related to the Kennedy assassination or not, Oswald actually had secretive contacts with the CIA or the FBI, or both." A visitor wearing a cowboy hat looks out onto Dealey Plaza from the Sixth Floor Museum located in the former Texas School Book Depository building in Dallas. AP Photo/LM Otero Sabato details several more intriguing connections to Oswald, including the top CIA officials who withheld information about Oswald after he allegedly showed up at the Cuban and Russian embassies in Mexico City on Oct. 9, 1963, in a failed attempt to secure visas so that he could go back to the Soviet Union.

When the CIA Mexico City station informed CIA headquarters that a man named Lee Oswald had been in contact with the Soviet consulate, Langley only sent a bare-bones reply with Oswald's basic facts. And the CIA, which had self-operating surveillance cameras and telephone bugs in both buildings, has never produced a photo or recordings of Oswald at either embassy.

"The pieces of the Oswald puzzle stamped CIA may be ill-fitting, but they could reasonably create a portrait of covert action," Sabato writes. "CIA headquarters might have found a good use for Oswald and would not have wanted to share how much they knew about this particular asset with lower-level employees or foreign country stations."

Taken together, Sabato concludes that the prime suspects for influencing Oswald to murder JFK would be the Mafia, the anti-Castro Cubans, or a rogue cell within the CIA.

"They all had means, motive, and opportunity," according to Sabato.

"As far as the CIA goes … it is clear beyond question that the CIA lied repeatedly to the Warren Commission and continued lying to the House Select Committee on Assassinations," Sabato told BI. "Revealing nothing about the assassination attempts on Fidel Castro. Revealing very little about the fact they kept close tabs on Oswald: They knew what he was doing, they were evaluating him. I think they had something in mind. I don't subscribe to the hidden coup within the CIA, although I don't rule it out. "

The suspicions about CIA went all the way to the top. Sabato writes that Marvin Watson, LBJ's postmaster general, told the FBI that "President Johnson expressed a belief in private in 1967 that the CIA had had a role in Kennedy's death."

Where the mystery stands

<strong><a href="http://www.amazon.com/The-Kennedy-Half-Century-Presidency-Assassination/dp/1620402807">Buy "The Kennedy Half-Century: The Presidency, Assassination, and Lasting Legacy of John F.Kennedy"</a></strong> Larry J. Sabato

"Given the lack of hard evidence, to accuse any arm or agency of the federal government of orchestrating Kennedy's assassination is both irresponsible and disingenuous," Sabato writes. "At the same time, it is impossible to rule out the possibility that small, secret cabal of CIA hard-liners, angry about Kennedy's handling of Cuba and sensing a leftward turn on negotiations with the Soviets ... took matters into their own hands lest the United States go soft on Communism."

The truth is that we may never know.

"I think this subject after 50 years requires some humility, which very few analysts of the assassination have," Sabato told BI.

"I am tired of reading books by authors who are absolutely certain that they have found the truth about the assassination. Malarkey," Sabato said. "There is no way to have a full picture. We are where we are and I think we are just going to have to accept that."

New details could come out soon, however, when thousands of documents are scheduled to be released in October 2017.

"The President at that time will get to rule whether anything can remain secret and redacted," Sabato said.