Now a new lawyer is on the case and he says his efforts have also failed.
"There is no doubt he does not remember the critical events," said William F. Pepper, the attorney who will argue for Sirhan's parole Wednesday. "He is not feigning it. It's not an act. He does not remember it."
Sirhan may not remember much about the night of June 4, 1968, but the world remembers.
They have heard how Sirhan was grabbed as he emptied a pistol in the crowded kitchen of the Ambassador Hotel here where Kennedy stood moments after claiming victory in the California presidential primary. They heard how he kept firing even as his hand was pinned to a table. They heard how Kennedy, the presumptive Democratic presidential nominee, was shot and died, changing the course of American history.
Parole Board members are bound to review those facts, but they won't consider the many conspiracy theories floated over the years.
Pepper, a New York-based lawyer who also is a British barrister, is the latest advocate of a second gunman theory. Believers claim 13 shots were fired while Sirhan's gun held only eight bullets and that the fatal shot appeared to come from behind Kennedy while Sirhan faced him.
Pepper also suggests Sirhan was "hypno-programmed," turning him into a virtual "Manchurian Candidate," acting robot-like at the behest of evil forces who then wiped his memory clean. It's the stuff of science fiction and Hollywood movies, but some believe it is the key.
How Pepper plans to use any of this to his client's advantage remains to be seen because it will have little bearing on the decision of the panel that must determine if Sirhan is suitable for parole. The board is not being asked to retry the case and lawyers may not present evidence relating to guilt or innocence.
At issue is whether Sirhan, 66, remains a threat to others or to himself, whether he has accepted responsibility for the crime and expressed adequate remorse and whether he has an acceptable parole plan if he is released.
His lack of memory makes expressions of remorse and accepting responsibility difficult.
Sirhan could address that if he speaks at the hearing at Pleasant Valley men's prison in Coalinga. Whether he'll do that is uncertain. He has rarely commented during 13 past parole hearings and sometimes hasn't shown up at all.
Pepper said in an interview with The Associated Press that he has had Sirhan examined several times by psychologist Daniel Brown of Harvard University, an expert in hypnosis of trauma victims. He will not disclose exactly what was accomplished in the sessions but said, "There have been substantial breakthroughs."
Pepper said he may have more to say after the hearing.
"It was very clear to me that this guy did not kill Bob Kennedy," said Pepper.
Asked who did kill the senator, he said, "I believe I have it but I'm not going to deal with it at this time."
In one of many emotional outbursts during his trial, Sirhan blurted out that he had committed the crime "with 20 years of malice aforethought," a statement that could now come back to haunt him. That and his declaration when arrested: "I did it for my country" were his only relevant comments before he said he didn't remember shooting Kennedy.
Public opinion could be an invisible force in the board's decision.
If Sirhan is released, he would be the first imprisoned political assassin to win parole in this country. James Earl Ray, convicted of killing the Rev. Martin Luther King Jr. and Jack Ruby, convicted of killing John F. Kennedy's assassin, Lee Harvey Oswald, both died in prison.
Sirhan was originally sentenced to death over objections by Kennedy family members who said they wanted no more killing. The sentence was commuted to life in prison when the U.S. Supreme Court briefly outlawed the death penalty in 1972.
Kennedy's son, Maxwell, who has spoken for the family previously, did not return phone calls from the AP regarding Sirhan.
The lawyer notes that he has a personal tie to Kennedy, having been chairman of his citizens' committee when he ran for Senate in 1964.
Pepper also represented James Earl Ray, through 10 years of appeals and a civil trial which he said proved that Ray was not King's killer. By then Ray was dead.
David Dahle, head Los Angeles deputy district attorney for parole candidates serving life sentences, said his remarks at the hearing will depend on what is presented by the defense.
"At this point, I am skeptical that I will see something that will cause me to not oppose the grant of parole," he said.
Few high profile prisoners have been released in the California system. Charles Manson and his followers have been repeatedly turned down for parole. Manson follower Susan Atkins attended her final parole hearing on a gurney dying of cancer but was denied release and died in prison three weeks ago.
Dahle said the board will review Sirhan's behavior in prison and whether the explosive outbursts of the young man who stood trial in 1969 have continued as he aged. By all accounts, Sirhan has been a model prisoner. But he said there will also be discussions of how he might adjust to life on the outside.
His brother, Munir Sirhan, 64, will submit a statement and a plan for Sirhan to live with him in his Pasadena home if released. However, even Pepper says that is an unlikely prospect because Sirhan, who was a Palestinian immigrant from Jordan, will be considered an illegal alien and would be turned over to immigration officials for deportation.
Munir Sirhan told The Associated Press he has made arrangements with a family in Jordan to house Sirhan if he is deported there.
"I hope it comes out in his favor," said Munir Sirhan. "As Christians we hold a lot of faith. I stand ready to help him in any way possible. If he is not deported our house is still here for him. We feel for the senator, God rest his soul. But 43 years is a long time. "
Both Pepper and Dahle said Sirhan's Middle Eastern connections have always provided a backdrop for considerations of parole.
"I don't think there will ever be a disconnect between issues of Middle East politics and this case," said Dahle.
Pepper said Sirhan is a victim of misperception because of his Palestinian Arab background. He said most assume Sirhan is a Muslim and some have referred to him as "the first terrorist." In fact, he said, Sirhan is a Christian and had no ties to terrorist groups.
Among those attending the hearing will be one of the victims. William Weisel, who was an ABC-TV director, was shot in the stomach.
"There's no doubt he was the shooter," Weisel said. "Whether or not there was another one, I don't know. If there were 13 shots, who was the other shooter?"
Having covered the White House through seven presidents, he said he does not ascribe to conspiracy theories because, "The government can't keep a secret."
However, Weisel said he will tell the parole board he has no objection to Sirhan's release "if the district attorney and the parole board decide it's to everyone's advantage."
Another surviving shooting victim, Paul Schrade, said he was not attending and would have no comment.