Gaeton Fonzi, one of the most relentless investigators on the House Select Committee on Assassinations in the late 1970s, whose final report to the panel concluded in 1979 that President John F. Kennedy “was probably assassinated as a result of a conspiracy,” died Aug. 30 in Palm Beach County. He was 76.

Fonzi died of complications of Parkinson’s disease at his home in Manalapan, his wife, Marie, said.

In Florida, Fonzi worked for Miami and Gold Coast magazines, writing investigative articles. He also wrote several other books, including a biography of the media mogul and philanthropist Walter Annenberg.

But the Kennedy assassination remained the story that consumed him, and former colleagues recalled the impatience he displayed in his pursuit of the story.

They called him Ahab.

Of course the assassination was a conspiracy, insisted Fonzi, a journalist recruited mainly on the strength of scathing magazine critiques he had written about the Warren Commission and its conclusion that Lee Harvey Oswald had acted alone in killing the president in Dallas on Nov. 22, 1963. But who were the conspirators? What was their motive? How could the committee close its doors without the answers?

Fonzi nailed those questions to the committee’s locked doors, figuratively, in a long article he wrote in 1980 for Washingtonian magazine and in a 1993 book, The Last Investigation. In both, he chronicled the near-blanket refusal of government intelligence agencies, especially the CIA, to provide the committee with documents it requested. And he accused committee leaders of folding under pressure — from congressional budget hawks, political advisers and the intelligence agencies themselves — just as promising new leads were emerging.

“Is it unrealistic to desire, for something as important as the assassination of a president, an investigation unbound by political, financial or time restrictions?” he asked in Washingtonian.

He never got the answer he had hoped for. Congress never authorized a follow-up to the work of the committee, which, from 1977 to 1979, also re-examined the assassination of Martin Luther King Jr., concluding that it, too, “likely” resulted from an unspecified conspiracy.

But historians and researchers consider Fonzi’s book among the best of the roughly 600 published on the Kennedy assassination, and credit him with raising doubts about the government’s willingness to share everything it knew. The author Jefferson Morley, a former reporter for The Washington Post, said The Last Investigation had refocused attention on a handful of reported contacts between CIA operatives and Oswald — tantalizing leads that had long been fascinating to conspiracy buffs but that had never been fully scrutinized by a veteran investigative reporter.

The CIA has denied that any such contacts occurred, and Fonzi spent most of his two years with the committee crisscrossing the world trying to prove otherwise. He considered it impossible that the CIA had never made contact with Oswald, a former Marine who defected to the Soviet Union in 1959, repatriated with his Russian wife and baby in 1962, and settled in Dallas, where he openly espoused Communist views.

“We called him Ahab, because he was so single-minded about that white whale,” said G. Robert Blakey, the chief counsel and staff director of the House committee, now a professor at the University of Notre Dame Law School. The white whale for Fonzi was the meaning of those supposed contacts.