United States Attorney General Robert Kennedy poses for a portrait in his Justice Department office circa 1964 in Washington, DC. Photo / Getty Images

Just before Christmas, Robert F Kennedy Jr pulled up to the massive Richard J Donovan Correctional Centre, a California state prison complex in the desert outside San Diego that holds nearly 4000 inmates. Kennedy was there to visit Sirhan B Sirhan, the man convicted of killing his father, Senator Robert F Kennedy, nearly 50 years ago.

While his wife, actress Cheryl Hines, waited in the car, Robert Kennedy Jr met with Sirhan for three hours, he revealed to the Washington Post last week. It was the culmination of months of research by Kennedy into the assassination, including speaking with witnesses and reading the autopsy and police reports.

"I got to a place where I had to see Sirhan," Kennedy said. He would not discuss the specifics of their conversation. But when it was over, Kennedy had joined those who believe there was a second gunman, and it was not Sirhan who killed his father.

"I went there because I was curious and disturbed by what I had seen in the evidence," said Kennedy, an environmental lawyer and the third oldest of his father's 11 children. "I was disturbed that the wrong person might have been convicted of killing my father. My father was the chief law enforcement officer in this country. I think it would have disturbed him if somebody was put in jail for a crime they didn't commit."

Kennedy, 64, doesn't know if his involvement in the case will change anything. But he now supports the call for a re-investigation of the assassination led by Paul Schrade, who also was shot in the head as he walked behind Kennedy in the pantry of the Ambassador Hotel in Los Angeles but survived.

Kennedy was 14 when his father died. Even now, people tell him how much Bobby Kennedy meant to them.

RFK's death - five years after his brother, President John F Kennedy, was gunned down in Dallas and two months after civil rights leader Martin Luther King Jr was killed in Memphis - devastated a country already beset by chaos.

In 1968, the Vietnam War raged, American cities had erupted in riots after MLK's assassination and tensions between war protesters and supporters were growing uglier. Robert Kennedy's newly launched presidential bid had raised hopes the New York Democrat and former attorney general could somehow unite a divided nation. The gunshots fired that June night changed all that.

Though Sirhan admitted at his trial in 1969 he shot Kennedy, he claimed from the start he had no memory of doing so. And midway through Sirhan's trial, prosecutors provided his lawyers with an autopsy report that launched five decades of controversy: Kennedy was shot four times at point-blank range from behind, including the fatal shot behind his ear. But Sirhan, a 24-year-old Palestinian immigrant, was standing in front of him.

Was there a second gunman? The debate rages to this day.

But the legal system has not entertained doubts. A jury convicted Sirhan of first-degree murder and sentenced him to death in 1969, which was commuted to a life term in 1972. Sirhan's appeals have been rejected at every level, as recently as 2016, even with the courts considering new evidence that has emerged over the years that as many as 13 shots were fired - Sirhan's gun held only eight bullets - and that Sirhan may have been subjected to coercive hypnosis, a real life "Manchurian candidate".

His case is closed. His lawyers are now launching a longshot bid to have the Inter-American Court of Human Rights hold an evidentiary hearing, while Schrade is hoping for a group such as the Innocence Project to take on the case. A spokesman for the Innocence Project said they do not discuss cases at the consideration stage.

In the final court rejection of Sirhan's appeals, US Magistrate Judge Andrew Wistrich ruled: "Even if the second shooter's bullet was the one that killed Senator Kennedy, [Sirhan] would be liable [for murder] as an aider and abettor." And if Sirhan was unaware of the second shooter, Wistrich wrote that the scenario of a second gunman who shot Kennedy "at close range with the same type of gun and ammunition as [Sirhan] was using, but managed to escape the crowded room without notice of almost any of the roomful of witnesses, lacks any evidentiary support".

On June 5, 1968, Kennedy had just won the California Democratic presidential primary and delivered a victory speech to a delirious crowd.

At 12.15am, the 42-year-old candidate and Schrade left the celebration, walking through the hotel pantry en route to a news conference. Schrade was a regional director of the United Auto Workers who had helped Kennedy round up labor support, and Kennedy had singled him out for thanks in his victory speech moments earlier.

Schrade, now 93, still recalls the scene in the pantry vividly.

"He immediately started shaking hands" with kitchen workers, Schrade said of Kennedy. "The TV lights went on. I got hit. I didn't know I was hit. I was shaking violently, and I fell. Then Bob fell. I saw flashes and heard crackling. The crackling actually was all the other bullets being fired."

Witnesses reported that Kennedy said, "Is everybody okay? Is Paul all right?"

Kennedy was still conscious as his wife, Ethel, pregnant with their 11th child, rushed to his side. He lived for another day and died at 1.44am June 6, 1968.

Schrade was shot above the forehead but the bullet bounced off his skull. Four other people, including ABC news producer William Weisel, were also wounded. All survived.

Sirhan was captured immediately; he had a .22-caliber revolver in his hand. Karl Uecker, an Ambassador Hotel maitre d' who was escorting Kennedy through the pantry, testified that he grabbed Sirhan's wrist and pinned it down after two shots and that Sirhan continued to fire wildly while being held down, never getting close to Kennedy. An Ambassador waiter and a Kennedy aide also said they tackled Sirhan after two or three shots.

Several other witnesses also said he was not close enough to place the gun against Kennedy's back, where famed Los Angeles coroner Thomas Noguchi found powder burns on the senator's jacket and on his hair, indicating shots fired at close contact. These witnesses provided more proof for those who insist a second gunman was involved.

Both the Los Angeles District Attorney's office and the Los Angeles Police Department declined interviews on what they consider a closed case.

Schrade believes Sirhan shot him and the others who were wounded but that he did not kill Kennedy. Since 1974, Schrade has led the crusade to try to persuade authorities - the police, prosecutors, the feds, anyone - to reinvestigate the case and identify the second gunman.

"Yes, he did shoot me. Yes, he shot four other people and aimed at Kennedy," Schrade, said in an interview at his Laurel Canyon home. "The important thing is he did not shoot Robert Kennedy. Why didn't they go after the second gunman? They knew about him right away. They didn't want to know who it was. They wanted a quickie."

At trial, defence lawyer Grant Cooper made the decision not to contest the charge that Sirhan fired the fatal shot and instead tried to persuade the jury not to impose the death penalty by arguing Sirhan had "diminished capacity" and didn't know what he was doing. It is a standard tactic by attorneys in death penalty cases, but Cooper, who died in 1990, was widely criticised for not investigating the case before conceding guilt.

Sirhan is now 74 and approaching 50 years behind bars. After California's courts abolished the death penalty in 1972, he was first made eligible for parole in 1986 but has been rejected repeatedly.

In 2016, Schrade spoke on Sirhan's behalf at his parole hearing and apologised for not coming forward sooner to advocate for Sirhan's release and exoneration.

California inmates are not permitted to give media interviews, and Sirhan did not respond to a letter from the Post. But his brother, Munir Sirhan, said Sirhan still holds out hope of being released and that his defense team probably hurt his case more than helped it.

There's plenty of damning evidence against Sirhan. He confessed to the killing at trial, though he claims this was done on his attorney's instruction. He took hours of target practice with his pistol earlier in the day, and he took the gun into the Ambassador that night. He had been seen at a Kennedy speech at the Ambassador two days earlier. He had a newspaper clipping critical of Kennedy in his pocket and had written "RFK must die" in notebooks at home, though he said he didn't remember doing that. And he waited in the pantry for about 30 minutes, according to witnesses who said he asked if Kennedy would be coming through there.

But questions about the case arose almost immediately in Los Angeles, resulting in hearings and reinvestigations as early as 1971 by the district attorney, the police chief, the county board of supervisors and the county superior court. Many of them focused on the ballistics of the case, starting with Noguchi's finding that Kennedy had been shot from behind, which Sirhan's lawyer didn't raise in his defence.

In addition, lead crime scene investigator DeWayne Wolfer testified at trial that a bullet taken from Kennedy's body and bullets from two of the wounded victims all matched Sirhan's gun.

But other experts who examined the three bullets said they had markings from different guns and different bullet manufacturers. An internal police document concluded that "Kennedy and Weisel bullets not fired from same gun," (Weisel was the wounded ABC news producer) and "Kennedy bullet not fired from Sirhan's revolver."

This prompted a Los Angeles judge in 1975 to convene a panel of seven forensic experts, who examined the three bullets and refired Sirhan's gun. The panel said no match could be made between the three bullets, which appeared to be fired from the same gun, and Sirhan's revolver. They found Wolfer had done a sloppy job with the ballistics evidence and urged further investigation.

In addition, witnesses said bullet holes were found in the door frames of the Ambassador pantry, and photos showed investigators examining the holes in the hours after the shooting. Between the three bullets that hit Kennedy and the bullets that hit the five wounded victims, Wolfer had accounted for all eight of Sirhan's shots. Bullets in the doors would indicate a second gun. Wolfer later said the holes and the metal inside were not bullets, and the door frames were destroyed after trial.

Though Los Angeles authorities had promised transparency in the case, the police and prosecutors refused to release their files until 1988, producing a flood of new evidence for researchers.

Among the material was an audiotape, first unearthed by CNN journalist Brad Johnson, which had been inadvertently made by Polish journalist Stanislaw Pruszynski in the Ambassador ballroom, and turned over to police in 1969.

Pruszynski's microphone had been on the podium where Kennedy spoke, and TV footage shows him detaching it and moving toward the pantry as the shooting happens.

In 2005, audio engineer Philip Van Praag said the tape revealed that about 13 shots had been fired. He said he used technology similar to the ShotSpotter technology used by police to alert them to gunshots, and which differentiates gunshots from firecrackers or other loud bangs.

Van Praag said recently different guns create different resonances and he was able to establish that two guns were fired, that they fired in different directions, and that some of the shot "impulses" were so close together they couldn't have been fired by the same gun. He said he could not say "precisely" 13 shots but certainly more than the eight contained by Sirhan's gun.

"There were too many bullets," Robert Kennedy Jr said. "You can't fire 13 shots out of an eight-shot gun."

British author Mel Ayton wrote The Forgotten Terrorist, which posits that Sirhan killed Kennedy because he supported sending military firepower to Israel - the Sirhans were Christian Palestinians forced from their Jerusalem home by Israel in 1948. He said Van Praag had misinterpreted the Pruszynski tape and that other experts who examined it show only eight "spikes," one for each gunshot. Ayton also cited numerous eyewitnesses who said they heard at most eight shots.

Ayton and investigative reporter Dan Moldea, who also wrote a book about the assassination, argue that Sirhan's gun could have reached Kennedy's back. No witnesses saw the actual shots fired in the chaos of the pantry, and Moldea noted that Kennedy almost certainly turned and tried to protect himself after the first shot, which some said was preceded by Sirhan yelling, "Kennedy, you son of a bitch!"

"What were Kennedy's last words?" Moldea asked during an interview. "'How's Paul?' How would Kennedy know Paul had been injured if he had not been turned around. He turned around when Sirhan rushes towards him, yelling 'you son of a bitch Kennedy.' Kennedy's not going to just stand there. He turns his back defensively."

Moldea theorised that Schrade fell forward into Kennedy, pinning him against a table and pushing him into the muzzle of Sirhan's gun, enabling him to fire four contact shots into Kennedy. One shot went through his jacket without hitting Kennedy, one went into his back and stopped below his neck, one went through his armpit and one went into his brain.

But Robert F Kennedy Jr doesn't find those theories persuasive. "It's not only that nobody saw that," Kennedy said. "The people that were closest to [Sirhan], the people that disarmed him all said he never got near my father."

Schrade used an expletive to describe Moldea's explanation and said he fell backward when he was shot above his forehead.

Both Ayton and Moldea assisted the California attorney general's office in contesting Sirhan's final appeal, and the government's legal briefs cited the investigative work of both men.

Moldea had initially been a believer in the second-gunman theory, but after interviewing numerous police officers, witnesses and Sirhan, he concluded in his 1995 book, The Killing of Robert F Kennedy, that Sirhan acted alone. He cited as additional proof a comment Sirhan reportedly made to a defence investigator about Kennedy turning his head before Sirhan shot him, a comment Sirhan strongly denied making.

More recently, Sirhan's lawyers have explored whether he was hypnotised to begin shooting his gun when given a certain cue, even hiring a renowned expert in hypnosis from Harvard to meet with Sirhan.

Judge Wistrich was completely dismissive of any suggestion of hypnosis. Schrade said the various theories of conspiracy and hypnotic programming are of little interest to him.

"I'm interested in finding out how the prosecutor convicted Sirhan with no evidence, knowing there was a second gunman," Schrade said.

It was Schrade who persuaded Robert F Kennedy Jr to examine the evidence. "Once Schrade showed me the autopsy report," Kennedy said, "then I didn't feel like it was something I could just dismiss. Which is what I wanted to do."

Kennedy called Sirhan's trial "really a penalty hearing. It wasn't a real trial. At a full trial, they would have litigated his guilt or innocence. I think it's unfortunate that the case never went to a full trial because that would have compelled the press and prosecutors to focus on the glaring discrepancies in the narrative that Sirhan fired the shots that killed my father."

Kennedy is not afraid to express controversial views. Last year, he and actor Robert DeNiro held a press conference to argue that certain vaccines containing mercury are unsafe for some children. He said he is not opposed to all vaccines, but wants to make them safer.

Three of his sisters - former Maryland lieutenant governor Kathleen Kennedy Townsend, human rights activist Kerry Kennedy and filmmaker Rory Kennedy - declined to discuss the assassination or the case against Sirhan. Kennedy understands why.

"I think that, for most of my family members," he said, "this is an issue that is still too painful to even talk about."

It's painful for him, too. Kennedy was asleep in his dorm at Georgetown Preparatory School in Bethesda, Maryland, on June 5, 1968, when a priest woke him and told there was a car waiting outside to take him to the family home, Hickory Hill, in McLean, Virginia. The priest didn't say why.

In his new memoir, American Values: Lessons I Learned from My Family, Kennedy said his mother's secretary was waiting for him. "Jinx Hack told me my father had been shot, but I was still thinking he'd be okay. He was, after all, indestructible."

Robert F Kennedy Jr, his older sister Kathleen and brother Joe flew to Los Angeles on Vice President Hubert Humphrey's plane, Air Force Two.

At Good Samaritan Hospital, Kennedy wrote, his father's head was bandaged and his face was bruised. A priest had already delivered last rites. His mother was there.

"I sat down across the bed from her and took hold of his big wrestler's hand," he wrote. "I prayed and said goodbye to him, listening to the pumps that kept him breathing. Each of us children took turns sitting with him and praying opposite my mom.

"My dad died at 1.44am, a few minutes after doctors removed his life support. My brother Joe came into the ward where all the children were lying down and told us, 'He's gone.' "