The convicted killer of a Nassau police officer has died in prison, after a parole board’s recent denial of his latest appeal for release, state correction officials said.

Authorities still are investigating the cause of death of John MacKenzie, 70, but a source with knowledge of the case said Thursday it’s believed to be suicide.

The Fishkill prison inmate had been convicted in the 1975 shooting death of Nassau police Officer Matthew Giglio, whose family repeatedly petitioned for MacKenzie to be kept behind bars.

State officials said he was pronounced dead at 7:04 a.m. Thursday.

“To a certain degree, this brings closure to the Giglio family,” Nassau police union president James Carver said Thursday. “They will never have to endure going through the parole appeal every two years. However, they still have to continue enduring the loss of their father and that will never go away.”

Parole officials repeatedly had denied MacKenzie’s appeals for release after he first became eligible in 2000 while serving a prison term of 25 years-to-life.

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He told Newsday in 2008 he wished he had received a death sentence because his parole prospects were grim. MacKenzie also said he took full responsibility for murdering Giglio and didn’t blame the officer’s family for wanting him to stay in prison.

“If it was my father or my family member, I’d probably feel exactly the same way,” said MacKenzie, who was 29 at the time of the crime.

Police Officer Matthew Giglio was killed by John Mackenzie in the 1975. Photo Credit: Nassau Co. Police handout / handout

MacKenzie’s daughter, Denise Peragine, 48, said in an interview Thursday that she’d talked to her father by phone Wednesday night and “he was very optimistic about a possible appeal” of his recent parole denial.

“He still had hope and his lawyer had hope,” Peragine said.

The daughter added that she’ll demand a full investigation of his death, and doesn’t believe the father she called a “compassionate, gentle, loving person” would take his own life.

“I need the public to know he’s not a monster. There’s another side to him,” she said.

Giglio’s family spoke out most recently in June to oppose MacKenzie’s release, saying he should never have a chance at freedom after “such a horrific act.”

MacKenzie shot Giglio, 35, in the chest after the officer responded to a burglary call at a West Hempstead boutique.

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The officer, an 11-year department veteran and father of three, was hospitalized for weeks before lapsing into a coma from which he never awoke.

In denying MacKenzie’s release most recently, the parole board noted he had “numerous letters of support,” a clean disciplinary record, and had gotten an education behind bars.

But the board also said in its decision that MacKenzie “showed little insight” into his criminal behavior during a July 26 interview.

“This panel remains concerned about your violent conduct in . . . shooting and killing a uniformed police officer, reflecting a callous indifference to human life,” the board said.

In May, a state Supreme Court justice found parole officials failed to cite a rational basis for repeatedly denying MacKenzie’s release and ordered the state to put together a new board to decide his case.

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But state officials said Thursday that the Board of Parole appealed that ruling, and the judge’s decision had been stayed pending the appeal’s outcome.

Nassau’s acting Police Commissioner Thomas Krumpter released a statement Thursday saying the department and the family could never get past the tragedy of the officer’s line-of-duty murder.