Former Mainichi Shimbun reporter Takichi Nishiyama, who was convicted in the 1970s after reporting on a secret pact between Japan and the United States over the 1972 return of Okinawa to Japanese rule, died of heart failure in Fukuoka Prefecture on Friday, his family said. He was 91.

As a political reporter for the major daily, Nishiyama suggested the existence of a secret bilateral pact in a newspaper article in 1971, a year before the island was returned to Japan after decades of U.S. administration.

He was arrested in 1972 on suspicion of instigating the leaking of state secrets by urging a Foreign Ministry official to pass on classified documents about the negotiation process behind the handover.

The Tokyo District Court found Nishiyama not guilty in 1974, but an appeals court overturned the ruling and convicted him with a suspended prison sentence. The guilty verdict was finalized at the Supreme Court in 1978.

File photo shows Mainichi Shimbun reporter Takichi Nishiyama (C) leaving the Tokyo Metropolitan Police Department upon his release in April 1972. (Kyodo)

Nishiyama quit the newspaper in 1974.

After official documents were discovered in the United States in the early 2000s that suggested a secret pact was made with Japan regarding the return of Okinawa to its rule, Nishiyama filed a damages suit against the state.

In the suit, filed with the Tokyo District Court in 2005, he claimed his career as a reporter was ruined by his wrongful indictment. The suit was rejected by the Supreme Court in 2008.

In 2009, Nishiyama and two dozen other people filed a lawsuit with the Tokyo District Court demanding that the government disclose documents concerning alleged bilateral pacts based on the public's right to know about the incident.

The watershed moment came during the trial when a former senior diplomat admitted for the first time in court that Japan and the United States had concluded a secret agreement on the cost burden for the Okinawa reversion.

"Japan shouldered $4 million in costs that Washington was supposed to pay to restore farmland in Okinawa (that had been used by U.S. forces)," 91-year-old Bunroku Yoshino, who was the Foreign Ministry's American Bureau chief, said in his testimony.

File photo shows Takichi Nishiyama during an interview in Kitakyushu, Fukuoka Prefecture, on April 4, 2022. (Kyodo)

In a landmark ruling in 2010, the district court said the government possessed the documents -- which showed there was a secret accord that Japan shoulder part of the U.S.' costs for the reversion of the southwestern island prefecture to Japanese rule-- and ordered it to disclose them. The state was also ordered to pay 100,000 yen ($730) in damages to each plaintiff.

In a 2011 ruling, however, the Tokyo High Court overturned the district court's decision, and the Supreme Court upheld the resolution of the appeals court in 2014.

After quitting his job with the Mainichi Shimbun, Nishiyama, a native of Shimonoseki, Yamaguchi Prefecture, worked for a fresh fruit company run by his relative in Kitakyushu, a city in Fukuoka Prefecture.

Recounting that period, Nishiyama later remarked he had felt "isolated, without any support." "I was shut out," he said.

He became actively engaged in discussions over national security and press freedom issues after suing the state in 2005.

In an interview with Kyodo News, Nishiyama explained why he did not report directly in his 1971 article on the existence of a telegram that suggested there was a secret bilateral pact.

"It was the best course of action to protect my news source," he said, adding that even if he had reported it, the government would not have admitted it existed anyway.

The telegram, however, ended up in the public domain after an opposition lawmaker grilled the government on the secret pact during a parliamentary session in 1972, holding in his hand a copy of the document Nishiyama had given him through an intermediary as he did so.

An ensuing investigation by the Foreign Ministry led police to arrest Nishiyama and the ministry official on suspicion of violating the National Civil Service Law. She was given a suspended sentence in 1974.

The saga has inspired several books and a TV drama. Nishiyama has been portrayed as a protagonist in Toyoko Yamasaki's novel "Unmei no Hito" (Person of Destiny).

Nishiyama himself was a prolific writer and published a book as recently as last year.

Nishiyama died at a care home in Kitakyushu, his family said Saturday.