Even after 50 years, keeping up with JFK assassination theories is tough.

You could say it's a lawn shot — especially if you're talking about the grassy knoll.

Last month, a known Cuban assassin was linked to the Nov. 22, 1963 slaying of the president in Dallas. Could he have been the long-rumored second gunman who fired from the infamous slope in Dealey Plaza?

Last week, an updated trajectory theory emerged, explaining how the fatal head shot might have come from the knoll, without contradicting medical evidence.

This weekend, a government scientist will keep the conspiracy chatter humming by reaffirming at a Dallas conference that not only is acoustics evidence sound, it indicates that five shots were fired – one of which was covered up by the House Select Committee on Assassinations.

Five shots could mean three gunmen.

That's a far cry from the most widely accepted theory: that Lee Harvey Oswald probably fired the only three shots from the Texas School Book Depository, a vantage point behind the presidential limousine.

Oswald's involvement is backed up by solid evidence (assuming none of it was faked): He worked at the depository, he ordered the bolt-action Italian rifle found on the sixth floor, his palm print was on it, three spent cartridges were found there, the "pristine" "magic bullet" recovered at the hospital and fragments found in the limousine matched that weapon, and afterward a cop was fatally shot with Oswald nearby.

And yet doubts persist that Oswald acted on his own. About 60 percent of Americans believe there was a conspiracy, according to a new Gallup poll.

Why the grassy knoll? The idea of a grassy knoll gunman began within minutes of the assassination, as more than a dozen police officers scoured the area behind a picket fence for a possible assassin and clues. Photographers quickly arrived and took pictures, but no suspicious character or weapon was clearly found on film or by police.

Eventually, more than 30 witnesses would say they heard at least one shot fired from the area — in front and to the right of the presidential limo — and shadowy shapes in photos and movies were interpreted as possible shooters.

When Abraham Zapruder's famous film of the assassination was released in 1975, the grassy knoll theory gained new momentum, as the simplest way to explain how the top right side of Kennedy's head burst open and his body arched backward and to the left. It looked to some as if the fatal bullet came from the right front, not the rear.

In 1979, the House Select Committee on Assassinations concluded "Kennedy was probably killed as a result of a conspiracy." The panel relied on analysis of a motorcycle cop's Dictabelt recording that suggested four shots — too many for Oswald to have fired fast enough.

The only-Oswald folks counter that: "ear" witnesses are often wrong; Kennedy's quick movement backward was either a kind of neurological reflex or the "jet effect" of matter exploding from his head; or that the Dictabelt's subtle artifacts weren't gunshots at all.

The hired gun from Cuba. Ease up on thinking "THIS MAN KILLED KENNEDY!" as a tabloid recently declared. Anthony Summers, a veteran journalist, did indeed name a possible grassy knoll shooter in Not in Your Lifetime, a comprehensive update on the assassination published last month. He also connects the highly capable suspect, Herminio Diaz Garcia, to both mob boss Santo Trafficante, who allegedly foretold Kennedy's slaying, and Oswald's killer, Jack Ruby. But Summers acknowledges the information came third-hand. "I've been cautious in the way that I reported it," he said by phone from Ireland.

The connection was made by Reinaldo Martínez Gomez, a friend of Diaz, who spoke in 2007 with Summers and Robert Blakey, ex-counsel to the House Select Committee on Assassinations. A friend, Tony Cuesta, told Martinez that Diaz confessed involvement while the two were together in prison, and another friend said, "Listen, the one who killed the President was our little friend. ... Herminio."

Martinez told Summers:

"I went there with Herminio five or six times ... and there was a waiting room where I sat while he went in to see Trafficante. On one of the visits, I noticed a particular man -- he caught my attention because in Cuba in July or August it is really hot, and this man was dressed in a double-breasted woolen suit and felt hat. [When] I asked Herminio, 'Chico, is he mad?', he told me it was Jack Ruby, a friend of Santo, who had come to see him." Martinez added:  "When I saw Ruby kill Lee Harvey Oswald in Dallas. . . . It looked like he still hadn't taken off the suit."

Since Diaz had assassinated a former police chief, and attempted to kill the president of Costa Rica and Cuban dicta­tor Fulgencio Batista, "there are reasons to think that if he did have some sort of participation it could have been as a shooter," Summers said.

Thus his speculation that Diaz might have fired from the grassy knoll.

Martinez sounded credible, Summers said: "He believed what he was saying and ... we could see no motive except to tell us the truth."

Unfortunately, Diaz died decades ago, and the men to whom he allegedly spoke are also deceased.

Shot from the knoll clipped Kennedy's skull? Last week, "Cold Case: JFK," an episode of PBS's Nova, performed ballistics tests and examined fracture patterns in autopsy x-rays to conclude that Kennedy was struck by two bullets fired from the rear, and that while a grassy knoll trajectory was possible, it's ruled out by the lack of a left side exit wound from JFK's skull.

(Read: Shooting holes in theory that a Secret Service agent killed President Kennedy)

But what if a grassy knoll shot didn't go through the skull, but plowed through it edgewise at the spot where a large piece of Kennedy's skull is missing, even after recovered pieces are accounted for?

That's the hypothesis of California private investigator Josiah Thompson, who was upset that Nova quoted him on camera but failed to fully discuss acoustic evidence that four shots may have been fired that day.

Such a tangential third shot might explain why Kennedy's head was thrown back and to the left, and how brain matter landed on two motorcycle cops and Secret Service agent Clint Hill, who were to the left of the limo at the time. If the fourth shot, less than a second later, struck the back of Kennedy's head, that would still explain the autopsy x-rays, as well as why Kennedy's head suddenly moves forward, around frame 328 on the Zapruder film, said Thompson, who taught philosophy at Haverford College before becoming a private investigator in the mid-1970s.

Thompson, author of "Six Seconds in Dallas," said he is nearing completion of a new book, "One Second in Dallas," focusing on the theory that the fatal head shot came from the grassy knoll, quickly followed by Oswald's final shot, hitting the back of Kennedy's head.

"There is no question whatsoever that he was hit in the back of the head," Thompson said.

The skull clipping idea may not be entirely new, judging from testimony by physicians to the House assassinations committee. According to Summer's Not in Your Lifetime:

Others thought the massive damage to the President's skull was perhaps the result of not one headshot but of two impacting almost simultaneously. Kennedy's personal physician, Admi­ral George Burkley, attended the autopsy and was to tell the Assassinations Committee that he "conceded the possibility" of two such shots. Dr. (Michael) Baden, head of the Committee's medical panel, acknowledged the "remote" possibility that the fatal head wounds "could have been caused by a shot from the grassy knoll, and that medical evidence of it has been destroyed by a shot from the rear a fraction of a second later.

Key to Thompson's argument is the Dallas police Dictabelt recording, and analysis that suggests the third shot came from the grassy knoll and coincides with the explosive head wound, when all four shots are synchronized to moments in the Zapruder film.

The evidence is tricky, though. "If you listen to the recording you cannot hear gunshots," said Nova producer Rushmore DeNooyer. If you've come across a YouTube video that purports to time the recording to the silent Zapruder film, realize any loud gunshot noises were dubbed in.

Experts had to infer gunshots through "statistical analysis of wave forms," he said.

The four-gunshot interpretation held sway with the House assassinations committee in 1979, a linchpin in its conclusion of conspiracy. Views have since been widely reported on both sides, with a National Academy of Sciences panel voting thumbs down in 1982; a science journal defense in 2001 by Don Thomas, an FDA entomologist and part-time Kennedy assassination buff; and another rejection in 2003 by researcher Michael O'Dell.

"There is no reasonable scientific basis for believing there are any shots on that recording, and certainly not one from the grassy knoll," O'Dell said via email.

Also dismissing the recording was a study commissioned by University of Virginia political scientist Larry Sabato for his new book, The Kennedy Half Century.

Thomas, though, not only defends the Dictabelt data, but ups the number of gunshots by one.

Five shots, three shooters? The plot thickens if Thomas is right. "My certainty that the shots are on the Dictabelt is undiminished," he emailed this week, explaining that from the beginning, back in the late 1970s, five shots were detected, including one from the grassy knoll. The House committee, however, had been leaning toward Oswald as the lone gunman, and found four easier to sell, he said.

After all, few witnesses apparently claimed to hear more than three.

Thomas, who mentioned five shots in his 2001 report, and in his 2010 book, Hear No Evil, will break it all down again at this weekend's November in Dallas conference.

Overlooking the fifth shot is one reason other analysts, including Sabato's, have gone astray, he said.

"The audio sequence of gunfire exactly matches the video sequence of wounding seen in the Zapruder film with the five-shot scenario. But that synch does not work if you throw out the fifth shot."

The first shot, fired from the depository, hit JFK with a fragment from a ricochet off the pavement, and happened at Zapruder frame No. 175, he said. The second shot, perhaps from another building behind the limo, missed. The third shot is the famous "single bullet theory" shot from the depository that hit Kennedy and Connelly. No. 4 was the explosive shot from the grassy knoll. The fifth, as Thompson said, might be the one that struck the back of Kennedy's head, producing the distinctive fracture pattern seen on the Nova special.

"The acoustical evidence is rock solid," Thomas said.

Summers offers a more neutral – and skeptical – perspective.

"It is evident that science — whether forensic, acoustic, or ballistic — has produced no certainties, and will not resolve the questions surrounding the Kennedy assassination," he writes in his book, Not in Your Lifetime.

The National Archives will release a supposedly final round of secret documents in 2017, although more than a thousand will probably remain classified by the CIA, according to the book.

"Full releases or no full releases, however, the lack of a real official will to investigate—long ago—means that outstanding mysteries about the assassination will never be resolved," according to Summers.

Thus the title: Not in Your Lifetime.

Contact staff writer Peter Mucha at 215-854-4342 or pmucha@phillynews.com.