William F. Pepper, who was the central figure in a decades-long effort to prove that the Rev. Dr. Martin Luther Jr. was killed not by a lone gunman but by a vast government plot, a controversial stance that made him something of a celebrity among the country’s teeming subculture of conspiracy theorists, died on April 7 in Manhattan. He was 86.

His wife, Mina Nguyen-Pepper, said the cause of his death, in a hospital, was pneumonia. He lived in Manhattan.

James Earl Ray shot and killed Dr. King in Memphis on April 4, 1968. He was arrested two months later at Heathrow Airport in London, just before boarding a flight to Brussels and eventually to Rhodesia (now Zimbabwe), which at the time was under white rule.

Mr. Ray pleaded guilty in order to avoid execution, and therefore did not go on trial. But he recanted soon after his conviction, and he spent the rest of his life claiming that he was innocent, himself a victim of a plot to kill Dr. King.

More than anyone else, it was Mr. Pepper, a lawyer, who kept Mr. Ray’s campaign alive long after Mr. Ray’s death in 1998. After taking on Mr. Ray as a client in 1988, he pressed the case across a variety of avenues, including courtrooms, the news media, a television special and three books.

Most experts on the King assassination dismissed Mr. Pepper’s contention, but he was repeatedly able to get a toehold in the mainstream. The televised special, a mock trial, was seen on HBO in 1993. Salon ran an excerpt from his 2017 book, “The Plot to Kill King: The Truth Behind the Assassination of Martin Luther King Jr.” He gave a talk at the National Civil Rights Museum that same year.

The mock trial, which featured a jury drawn from around the country but which was largely stage-managed by Mr. Pepper, found Mr. Ray not guilty. In 1999, Mr. Pepper represented members of Dr. King’s family in a successful wrongful-death suit against Loyd Jowers, convincing a jury of their assertion that Mr. Jowers had hired a retired police officer as the real assassin.

But Mr. Pepper’s arguments rarely stood up under scrutiny. Five government investigations and a long list of historians and journalists concluded that Mr. Ray had acted alone. Among them was Hampton Sides, who detailed the hunt for Dr. King’s killer in “Hellhound on His Trail: The Electrifying Account of the Largest Manhunt in American History” (2010).

At best, they concluded, Mr. Ray may have received some financial support from white supremacists eager to see Dr. King dead — far from the sorts of conspiracies that Mr. Pepper alleged.

He proposed a vast network of plotters, including the F.B.I., President Lyndon B. Johnson and the Mafia. In his 1995 book, “Orders to Kill: The Truth About the Murder of Martin Luther King,” he claimed that a man named Billy Ray Eidson led a Special Forces team to Memphis to kill Dr. King, and that Mr. Eidson was in turn killed to keep the mission a secret.

But when “Turning Point,” an ABC News program, investigated Mr. Pepper’s claims in 1997, it found Mr. Eidson alive and well — and brought him on the show to confront Mr. Pepper.

“We found not a single bit of verifiable evidence to support Mr. Pepper’s theory,” Forest Sawyer, a correspondent for the program, told The New York Times. “I emphasize ‘verifiable,’ because Mr. Pepper says he has other sources he’ll reveal at a later date.”

Mr. Eidson then sued Mr. Pepper. He won $11 million from him and his publisher in 1997.

Nor was the civil case against Mr. Jowers very persuasive to the reporters who covered the trial. Members of the jury and even the judge fell asleep at times, unsworn testimony was allowed as evidence, and Mr. Jowers himself was never called to testify, except through a deposition.

The King family received damages of $100.

William Francis Pepper was born on Aug. 16, 1937, in the Bronx to Irish immigrants. His father, Francis, worked for a railroad, and his mother, Lillian (Gilliland) Pepper, managed the home.

He received a bachelor’s degree from Columbia University in 1959, a doctorate in education from the University of Massachusetts at Amherst in 1973 and a law degree from Boston College in 1977.

After graduating from Columbia, he was an instructor in political science at Mercy College in Westchester County, N.Y. He also became active in the antiwar movement. He traveled to South Vietnam as a freelance journalist in 1966, returning with a harrowing account of the toll of napalm bombs on women and children.

That account ran in the countercultural magazine Ramparts. Mr. Pepper claimed it was his article that persuaded Dr. King to come out publicly against the war, and he was in the audience at Riverside Church in Manhattan on April 4, 1967, when Dr. King gave a resounding speech condemning America’s involvement in Southeast Asia.

Mr. Pepper became the executive director of the National Conference for New Politics, which met in Chicago in late 1967 to consider, among other things, running Dr. King and Benjamin Spock, the pediatrician and antiwar activist, as a third-party presidential ticket.

But the conference collapsed amid acrimony between liberal civil rights figures like Dr. King and Black militants, who Mr. Pepper later maintained were actually government saboteurs.

Mr. Pepper first met Mr. Ray in 1977, when he and Ralph Abernathy, who had been a close ally of Dr. King’s, visited him in prison. Both left the meeting convinced that Mr. Ray was not a killer.

Still, it would be another decade before Mr. Pepper took him on as a client. Meanwhile, he had moved to London, where he practiced commercial law on behalf of governments in the Middle East and Asia.

Mr. Pepper married Mina Nguyen in 2014. Along with her, his survivors include their daughter, Lilly.

The King assassination was only one of Mr. Pepper’s interests. He was a frequent presence at conspiracy-theory conventions and a fellow traveler among 9/11 truthers. In 2011, he argued for parole and a new trial for Sirhan Sirhan, who was convicted of killing Senator Robert F. Kennedy in Los Angeles in 1968, claiming that Mr. Sirhan had been framed.

According to Mr. Pepper, Mr. Sirhan had been hypnotized and “programmed” to fire diversionary shots, while another assassin did the actual killing.

“Sirhan was set up to be the distracting actor,” Mr. Pepper told CNN in 2012, “whilst the shooter bent down close behind Bob and fired close and upward, with four bullets hitting the senator’s body or passing through his clothing.”

Mr. Pepper’s motion was rejected, and Mr. Sirhan remains in prison.

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