In a stunning revelation from behind bars, a Louisiana man who was cleared of the murder of Maria Marshall by the side of the Garden State Parkway nearly 30 years ago has now admitted to being the triggerman, authorities said.

But since Larry Thompson, 71, was already cleared once by a New Jersey jury, he can’t be prosecuted in the infamous slaying of Maria Marshall, who was shot twice in the back in the early hours of Sept. 7, 1984, authorities said. Marshall's husband, Robert Marshall, was convicted for arranging his wife's murder, and is still in prison.

Thompson admitted on April 25 that he was in the state at the time of the crime – telling the retired investigator who led the hunt for the killer that he was the triggerman as detectives had believed, said Joseph Coronato, the Ocean County Prosecutor.

Marshall was found dead in the passenger seat of her husband’s white Cadillac at the Oyster Creek Picnic Area, authorities said at the time. The investigation later turned up that she had been shot twice at close range by a .45-caliber automatic pistol.

Her husband, Robert O. Marshall, told detectives the Cadillac had a problem with the right rear tire of the car, pulled over to the picnic area, and when he got out to check the car, was knocked unconscious from behind. He awoke to find his wife shot dead, he said.

Robert O. Marshall, in his current picture on file with the state Department of Corrections

But after a three-month investigation, Robert Marshall and three men from Louisiana were indicted: Billy Wayne McKinnon, Robert A. Cumber and Thompson.

The murder drew huge attention – and the trial had to be moved south to Atlantic County. After a two-month trial, Marshall was found guilty of offering money to people to murder his wife – and received the the death penalty, authorities said. The death sentence was overturned in 2004 and Robert Marshall is now serving 30 years to life in prison.

But Thompson was found not guilty, largely based on the testimony of his family members who said he had been seen in Louisiana around the time of the murder.

At least one of those family members recanted their testimony to investigators in April, which led to Thompson’s admission, authorities announced today.

"There are no legal avenues we can pursue at this time because our laws preclude further prosecution and substantial time has lapsed since the event occurred, we felt it was important to publicize these developments so that the family and friends of Maria Marshall could find some degree of closure to this tragic event," Coronato said.

The killing spawned the best-selling book "Blind Faith" and a television miniseries — but the person who had pulled the trigger had never been identified.

Chief James A. Churchill, who led the Ocean County Prosecutor’s investigation of the Marshall death nearly 30 years ago, got the confession from Thompson on April 25. The retired investigator was contacted in October 2013 by the Caddo Parsish District Attorney’s Office. Louisiana authorities told him Thompson was suspected in a killing in Shreveport – a 1979 murder that sounded familiar, considering the victim was with her husband at the time she was killed. Due to the similarities in the case, Churchill was invited down to assist.

The revelations began to come. On April 14, Thompson admitted to the Caddo District Attorney that he had used a false alibi at his trial in 1986 for the Marshall murder, authorities said today. Three days later, Thompson's son Brian, who had said in 1986 that his father accompanied him to the dentist around the time of the killing, changed his story, saying it was his mother who had taken him, authorities said.

Finally, on April 25, in Angola state prison in West Feliciana Parish, Thompson admitted his full involvement, authorities said. To Churchill - the man who had pursued him but failed nearly 30 years ago - he reportedly admitted that he was in New Jersey at the time of the killing – and that he fired the shots that killed Maria Marshall at the picnic area off the Parkway.

There were further admissions, authorities said. The alibi about the dental appointment was created while he was in New Jersey awaiting trial – and was spurred by the fact that Thompson's wife had signed her husband's name to the bank slip used to withdraw the cash to pay for the dental appointment The other witnesses providing his alibi were either lying or mistaken, Thompson also said.

But Thompson won’t serve any time for killing Maria Marshall, despite his admission, according to authorities. Thompson, currently in the 12th year of a 50-year-plus sentence for armed robbery and attempted murder, cannot be prosecuted. The U.S. Constitution guarantees that a person cannot be tried twice for the same crime. Since he was found “not guilty” in 1986, he can’t be subject to “double jeopardy,” Coronato said.

The witnesses who lied on his behalf also can’t be charged, since the statute of limitations for perjury in New Jersey is 5 years, Ocean County authorities added.

Thompson has also now been indicted in that 1979 killing in Louisiana, according to Shreveport authorities. The inmate is now indicted in the murder of Deanna Montgomery - as has Montgomery's husband James Montgomery, authorities announced Thursday. The mother of two young children was killed on New Year's Day by a shotgun blast to the head, authorities allege.

However, there is at least some degree of closure on the Marshall case, according to Al Della Fave, a spokesman for Coronato.

"The law enforcement community, particularly those who worked on the investigation and prosecution of Larry Thompson, are comforted with this news as it confirms what we all believed to be the facts 30 years ago," Della Fave said.

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