Mark Lane, the defense lawyer, social activist and author who concluded in a blockbuster book in the mid-1960s that Lee Harvey Oswald could not have acted alone in killing President John F. Kennedy, a thesis supported in part by the House Select Committee on Assassinations in 1979, died on Tuesday at his home in Charlottesville, Va. He was 89.

The cause was a heart attack, his friend and paralegal Sue Herndon said.

The Kennedy assassination, one of the manifest turning points of the 20th century, was the pivotal moment in Mr. Lane’s life and career. He would go on to raise the possibility of conspiracy in the assassination of the Rev. Dr. Martin Luther King Jr. five years later, but it was his Kennedy inquiry that made his name.

Before the president’s murder on Nov. 22, 1963, Mr. Lane was a minor figure in New York’s legal and political circles. He had organized rent strikes, opposed bomb shelter programs, joined the Freedom Riders, took on civil rights cases and was active in the New York City Democratic Party. He was elected a State Assemblyman in 1960 and served one term.

After the Kennedy murder, Mr. Lane devoted much of the next three decades to its investigation. Almost immediately he began the Citizens’ Committee of Inquiry, interviewed witnesses, collected evidence and delivered speeches on the assassination in the United States and in Europe, where he befriended Bertrand Russell, the British philosopher, who became an early supporter of Mr. Lane’s efforts.