TOMS RIVER, N.J. - At a picnic area on the Garden State Parkway, Robert Marshall pulled over shortly after midnight to check a tire on his white Cadillac. He was walking to the rear of the car, he said, when he was struck on the head and knocked unconscious.

When he came to, Marshall said, he found his wife, Maria, dead in the front seat of the car, with two bullet wounds to her back. She had been shot with a .45-caliber automatic pistol.

The story didn't add up, and Marshall would be convicted of hiring his wife's killers.

But for the next 30 years, the man accused of pulling the trigger remained silent about the case, which gained national attention through a best-selling novel.

Now, he has confessed. But Larry Thompson, 71, who is serving a sentence of more than 50 years for his part in an armored-car robbery and attempted killing of a police officer during an ensuing car chase in Louisiana, can't be retried for the fatal 1984 shooting, authorities said, because of the constitutional prohibition against double jeopardy.

"Though 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 we publicize these developments so that the family and friends of Maria Marshall could find some degree of closure to this tragic event," Ocean County Prosecutor Joseph D. Coronato said in a statement Friday.

Thompson had been accused of murdering Maria Marshall, who was shot about 12:30 a.m. Sept. 7, 1984, in the Oyster Creek picnic area of the Garden State Parkway in Lacey Township.

Thompson's wife, Wanda; son Brian; and brother Steven falsely testified during the trial that they had seen him in Louisiana on or about the date of the shooting, authorities said.

The killing became the focus of Joe McGinniss' bestseller, Blind Faith.

The confession "means a lot to me," said James Churchill, who in 1984 was the lieutenant in charge of the major crimes unit of the Ocean County Prosecutor's Office and supervised the Marshall investigation. He retired as the chief.

"When I went down there," to Louisiana to interview Thompson, "the Caddo Parish assistant district attorney said, 'You can cross this off your bucket list,' and that's basically how I characterize it.

"I wanted this guy to tell me that he shot her and that his alibi witnesses were lying or mistaken," he said. "This gives closure to Maria Marshall's family."

Churchill reached one of the Marshalls' three sons, Christopher, of Bethlehem, Pa., to tell him the news.

"He was emotional and thanked me," he said. "He was going to call his brothers. He said, 'It gives you guys some closure, too.' "

Robert Marshall, who was 45 in 1984 and owned an insurance and estate planning business, was accused of hiring Thompson, then 41, of Fairview Alpha, La., and Billy Wayne McKinnon, then 41, of Shreveport, La., to murder his wife of 20 years.

He told his mistress that he wanted to "get rid of" Maria Marshall and that her death would provide him with enough insurance money to get out of debt, she testified in 1986.

The conversation, said Sarann Kraushaar, who admitted having a 14-month affair with Marshall, took place several months before the killing.

"I didn't think he was serious," Kraushaar, a former high school vice principal, said at the time. "I told him the idea was absurd and out of the question."

McKinnon, who became a prosecution witness, said Marshall agreed to pay him $65,000 to arrange the murder of his wife.

Marshall mailed at least one money order from Toms River using the name James McAllister and the return address of 400 N. Broad St., Philadelphia - at that time the address of The Inquirer.

In return, McKinnon said, he brought Thompson to New Jersey, and Thompson staged the robbery and murder.

Marshall was found guilty of offering money or the promise of money to individuals for the murder of his wife, and sentenced to death by lethal injection.

He fought the outcome and was resentenced in 2006 to life in prison, with the possibility of parole in eight years. He is to receive his first parole hearing on Dec. 14.

Until his removal from New Jersey's death row, Marshall had been the longest-serving inmate there since the state reinstated the death penalty in 1982.

Thompson was also charged in the killing, but was exonerated because of the false alibi. He returned to Louisiana, where he became involved in other criminal activity. He has been incarcerated at Louisiana State Penitentiary for the last 12 years.

Ocean County authorities were called to Louisiana in 2013 to help with an investigation of a Shreveport murder involving the shooting of a woman who was with her husband at the time. Thompson was believed to be involved. Churchill was invited to assist because of its similarities to the New Jersey case.

By April, Thompson had acknowledged the false alibi in the Ocean County shooting, and his son admitted giving "inaccurate" information about his father's whereabouts at the time.

At the prison where Thompson was held, Churchill quizzed him again.

"I asked, 'Did you fire those shots that caused the death of Maria Marshall?' "

"He said, 'Yes, I did,' " Churchill recalled.