Sirhan Sirhan, the assassin of Democratic presidential candidate Robert Kennedy in 1968, was denied parole for the 15th time on Wednesday, a California prison official said.

Sirhan, 71, had a suitability hearing at the Richard J. Donovan Correctional Facility near San Diego, the California Board of Parole Hearings said on its website.

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Sirhan will be up again for parole in five years, said Luis Patino, a spokesman for the California Department of Corrections and Rehabilitation.

The Palestinian-born Sirhan is serving a life sentence for gunning down Kennedy, 42, at the Ambassador Hotel in Los Angeles on June 5, 1968. The shooting occurred minutes after the U.S. senator from New York and former U.S. attorney general gave his victory speech after winning the California Democratic primary. Kennedy died the next day.

Sirhan was sentenced to death in 1969. His sentence was commuted to life in prison after California banned the death penalty.

Paul Schrade, a 91-year-old Kennedy confidant and among the five people wounded in the shooting, told the board that Sirhan should be granted parole since evidence showed that a second gunman killed Kennedy.

"The evidence clearly shows you were not the gunman who shot Robert Kennedy," Schrade said in remarks prepared for the hearing.

Sirhan fired in front of Kennedy but the candidate was struck in the back by three bullets, including a fatal shot to the back of the head, Schrade said.

An analysis of an audio recording shows that 13 shots were fired, but Sirhan's gun held only eight rounds and he had no chance to reload, Schrade said.

Schrade alleged misconduct in the investigation by the Los Angeles Police Department and called for a new probe of Kennedy's killing.

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A federal judge last year rejected similar arguments by Sirhan's lawyers, who had sought to have him released, saying he was innocent.

Sirhan has said that he had no recollection of the killing, although he has also said he had fired at Kennedy because he was enraged by his support for Israel.

Sirhan was last denied parole in 2011. He is imprisoned in San Diego.

Kennedy's older brother, President John F. Kennedy, was assassinated in 1963.