John Thompson, who spent 14 years on death row in Louisiana before being exonerated in a murder case that brought national attention to prosecutorial misconduct and the perils of wrongful conviction, died Oct. 3 at a hospital in New Orleans. He was 55.

The cause was a heart attack, said Emily Maw, the director of Innocence Project New Orleans, a nonprofit law office she said collaborated with Mr. Thompson on justice initiatives after his 2003 release from prison.

Mr. Thompson was a 22-year-old self-described "small-time weed dealer" in 1985, when his legal nightmare began.

That January, he was arrested, after authorities received a tip, for the murder weeks earlier of Ray Liuzza Jr., a New Orleans hotelier. His face splashed across the news, Mr. Thompson was then identified as the alleged assailant in an unrelated carjacking.

Prosecutors put him on trial first for the carjacking, winning a conviction in April 1985. The next month, he was convicted in the murder case.

In large part because of the prior carjacking conviction, prosecutors successfully argued Mr. Thompson should receive the death penalty.

During the years-long legal drama that followed — a time Mr. Thompson spent in an isolation cell 23 hours a day — a raft of evidentiary problems emerged.

At the murder trial, co-defendant Kevin Freeman blamed the crime on Mr. Thompson, who had in his possession Liuzza's ring and the gun used in the killing. Mr. Thompson's legal team made the strategic call that he not testify in his defense, to avoid possible cross-examination about the car-jacking conviction.

The decision not to testify meant Mr. Thompson could not explain how he came to have the ring and gun: "A few weeks earlier [Freeman] had sold me a ring and a gun; it turned out that the ring belonged to the victim and the gun was the murder weapon," Mr. Thompson wrote in an op-ed published in the New York Times in 2011.

(Elsewhere, Mr. Thompson is said to have received the two items in a drug transaction.)

As for the car-jacking conviction, a private investigator discovered — just weeks before Mr. Thompson's seventh and final scheduled execution date — a 1985 lab report revealing the apparent carjacker had Type B blood.

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Mr. Thompson was Type O.

As controversy mounted in his case, a lawyer came forward to say his friend, a former prosecutor by then deceased, had confessed on his deathbed he had concealed the lab report to more easily convict Mr. Thompson. A 1963 U.S. Supreme Court ruling in Brady v. Maryland bars prosecutors from denying exculpatory evidence to a defendant.

In 1999, New Orleans District Attorney Harry Connick Sr. (the father of singer Harry Connick Jr.) joined Mr. Thompson's lawyers in requesting a stay of execution. The stay was granted, a new trial was ordered, the carjacking charge was eventually dropped, and Mr. Thompson was removed from death row.

He received a new murder trial and, in 2003, was acquitted. "You forget about the beauty of small things," he told the publication the Gambit in 2003, describing the disorientation of freedom after so many years of incarceration. "You've been gone from them for so long."

After his release, Mr. Thompson brought a civil suit against Connick's office, winning damages of $14 million.

In 2011, the Supreme Court dismissed the award in a 5-4 decision.

Justice Clarence Thomas, who wrote the majority opinion, found Mr. Thompson could not show "deliberate indifference" by the prosecutor's office.

Justice Antonin Scalia, writing in a concurring opinion, observed the injustice done to Mr. Thompson was to be blamed on a single "miscreant prosecutor," not on the office overall.

Justice Ruth Bader Ginsburg took the rare step of reading from the bench her dissent, in which she argued Mr. Thompson had been subjected to a "gross, deliberately indifferent and long-continuing violation of his fair trial right."

Responding to the court's decision, Mr. Thompson told the Times, "If I'd spilled hot coffee on myself, I could have sued the person who served me the coffee . . . but I can't sue the prosecutors who nearly murdered me."

After his release, Mr. Thompson helped start Resurrection After Exoneration, an organization that helps former inmates readjust to life outside bars.

"Men come home, and the system has nothing in place to help them put their lives back together," he observed. "They need to be reprogrammed because the survival tactics they learned in prison don't work in the outside world."

John Colin Thompson was born in New Orleans on Sept. 6, 1962. His father was in and out of prison, and Mr. Thompson was raised largely by a grandmother.

After his release from prison, he married Laverne Thompson. Besides his wife, survivors include two sons from earlier relationships, Dedric West and John Thompson Jr.; his mother, Josephine Casby; four stepchildren; two half-brothers; a half sister; and 12 grandchildren.

When he was in prison, Mr. Thompson was said to have spent much of his time reading legal writings and listening to the radio.

He reportedly sold his radio and typewriter so his son could participate in a school trip. In his Times op-ed, he recalled his son learned of what was to be Mr. Thompson's final execution date at school, when a teacher read to the class a newspaper article about his case.

"She didn't know I was John Jr.'s dad; she was just trying to teach them a lesson about making bad choices," Mr. Thompson wrote. "So he learned that his father was going to be killed from his teacher, reading the newspaper aloud."

He described the death penalty as evil, and said he opposed it for his children, or for anyone else who might one day find him or herself wrongly accused of a crime.

"The same thing could happen to them," Mr. Thompson said on CNN in 2014. "A person could have that much power over your life without being [held] accountable for it. I can throw the evidence away and still try to kill you. Whether you did it or not, without no consequences. That's scary. That's should be scary to everybody in the whole world."