In a 1966 photo, Mark Lane shows the route followed in Dallas by President John F. Kennedy on Nov. 22, 1963. (Card/AP)

Mark Lane, a crusading lawyer for often unpopular causes, who was best known as an early and persistent skeptic of the lone-gunman account of the assassination of President John F. Kennedy and who wrote several books affirming his belief that the president was the victim of a far-reaching government conspiracy, died May 11 at his home in Charlottesville. He was 89.

The cause was heart disease, said his wife, Patricia Lane.

In addition to his decades-long interest in the Kennedy assassination, Mr. Lane was a participant in other high-profile events, with clients including Vietnam War protesters; American Indians in the 1973 uprising at Wounded Knee, S.D.; and Jim Jones, the leader of the 1970s cult that came to an end with a mass suicide in South America.

For a time, Mr. Lane represented James Earl Ray, the convicted killer of the Rev. Martin Luther King Jr., alleging that he was an innocent pawn in a government plot.

Once called “the country’s most controversial legal gadfly” by Newsweek magazine, Mr. Lane was also a vice presidential candidate in 1968 on a fringe-party ticket headed by comedian and activist Dick Gregory.

Mark Lane, second from left, at a 1970 press conference with antiwar activists, including actress Jane Fonda, right. (James McNamara/The Washington Post)

Widely admired in some circles and reviled in others, Mr. Lane seemingly saw conspiracies lurking in every dark corner of American society. Even as he charged the news media with being manipulated by the government and big business, he appeared on hundreds of television shows, was accessible to reporters and published 10 books.

Throughout Mr. Lane’s career, the Kennedy assassination was a subject he could not leave alone. His involvement in the story started soon after the president was shot to death in Dallas on Nov. 22, 1963.

Within two months, he became the legal representative of the mother of Lee Harvey Oswald, the suspected assassin who was slain in a Dallas police station by nightclub owner Jack Ruby two days after the Kennedy assassination.

Mr. Lane testified before the Warren Commission, the official federal inquiry into the assassination, but disputed its findings when the final report was released in 1964. Two years later, Mr. Lane published his first book, “Rush to Judgment,” which criticized the Warren Commission’s methods and conclusions. The book became an immediate bestseller and helped fuel what became a burgeoning industry of elaborate theories of conspiracy and secrecy surrounding the Kennedy assassination.

Among other things, Mr. Lane maintained that Oswald could not have acted alone. He speculated that at least one other gunman may have fired the fatal shots from a “grassy knoll” as Kennedy’s open-air limousine passed through Dealey Plaza in Dallas. Mr. Lane also voiced doubt about another central conclusion of the Warren Commission, that a single bullet had gone through Kennedy’s body and struck Texas Gov. John B. Connally Jr., who was sitting in the front seat.

Moreover, Mr. Lane believed that Ruby, who killed Oswald on national live television, was part of a larger conspiracy that included law enforcement officials.

The influence of “Rush to Judgment” contributed to a pervasive skepticism about the Kennedy assassination. By the 1970s, a majority of Americans came to believe that some sort of conspiracy led to the president’s death, but there has been little agreement about how it may have occurred.

Mr. Lane charged that the Warren Commission worked backward from the conclusion that Oswald was killer, ignoring any evidence to the contrary. In time, critics would accuse Mr. Lane of the same tactics, only in reverse, saying he dismissed any suggestion that Oswald acted on his own.

In subsequent books, most notably in 1991’s “Plausible Denial: Was the CIA Involved in the Assassination of JFK?”, Mr. Lane wrote that the CIA was angry that Kennedy had abandoned plans to overthrow Fidel Castro in Cuba and feared that the president would eventually shut down the spy agency altogether. He charged that the conspiratorial network included convicted Watergate co-conspirator E. Howard Hunt.

By the time he published his fourth book about the assassination, “Last Word: My Indictment of the CIA in the Murder of JFK,” in 2011, Mr. Lane had removed the question mark from his assertions about the CIA. Other researchers criticized his conclusions as fanciful and not supported by the facts.

Vincent Bugliosi, the true-crime author and onetime prosecutor of Charles Manson, who spent years writing a comprehensive study of the Kennedy assassination, “Reclaiming History,” called Mr. Lane “unprincipled” and “a fraud.”

Throughout his career, Mr. Lane often claimed that government conspiracies were linked to many significant events in U.S. history. He was arrested at many antiwar demonstrations, often with actress Jane Fonda. In a controversial 1970 book, “Conversations With Americans,” Mr. Lane alleged that members of the U.S. military committed countless war crimes in Vietnam.

He was a popular speaker on college campuses in the 1960s and 1970s and was caught up in many causes of the day. In 1974, he was jailed in connection with the trial of several American Indians who had seized control of the town of Wounded Knee. He later helped win the acquittal of Indian activists Russell Means and Dennis Banks.

In 1977, Mr. Lane appeared before a congressional panel investigating the assassinations of the 1960s as the attorney for Ray, King’s convicted killer. Mr. Lane said Ray was innocent and charged the FBI with being the “prime suspects” in King’s murder.

Mr. Lane drew an unusually sharp rebuke from the committee, which noted in a report that his statements were “based on little more than inference and innuendo.”

One of the strangest episodes in Mr. Lane’s career came in 1978, when he was hired by the self-proclaimed Rev. Jim Jones to represent his “Peoples Temple.” Mr. Lane suggested that Jones and his followers — which he called “an incredible experiment with such vast potential for the human spirit” — were being harassed by the government.

He was in Jonestown, Guyana, when Rep. Leo J. Ryan Jr. (D-Calif.) and other members of a U.S. delegation were killed by several of Jones’s supporters on Nov. 18, 1978. The same day, an estimated 900 people, including Jones, died after drinking cyanide-laced Kool-Aid. Mr. Lane said sympathetic members of the cult helped him escape with his life into the surrounding jungle.

Mark Lane was born Feb. 24, 1927, in New York City. His father was an accountant.

Mr. Lane served in the Army after World War II, attended Long Island University and graduated in 1951 from Brooklyn Law School. He practiced law before serving one term in the New York state legislature in the early 1960s.

During the civil rights era, Mr. Lane was among the first Freedom Riders to venture into the South, and he was arrested for attempting to integrate public restrooms in Mississippi.

Mr. Lane lived in Washington for many years before settling in Charlottesville about 10 years ago. His marriages to singer Martha Schlamme and to Anne-Marie Dabelsteen ended in divorce.

Survivors include his wife of 30 years, Patricia Erdner Lane of Charlottesville; two daughters from his second marriage; a daughter from another relationship; and four grandchildren.

In one of his lesser-known books, “Arcadia” (1970), Mr. Lane championed the case of James Richardson, an African American man convicted by an all-white jury in rural Florida of killing his seven children by poisoning.

Richardson languished in prison for 21 years before Mr. Lane and Florida lawyer Ellis Rubin argued for his release. A special prosecutor — future U.S. attorney general Janet Reno — agreed that Richardson did not receive a fair trial and should be freed.

It was the happiest moment of Mr. Lane’s career, his wife said in an interview.

“You’re free, James!” Mr. Lane said, helping to win Richardson’s release in 1989. “You’re free!”

“I still don’t believe it,” Richardson said. “You got to put me on the ground, put me on the ground, where I can feel the dirt, feel the breeze.”