Shigeaki Mori, a Hiroshima atomic bomb survivor and historian who became known internationally for identifying American POWs killed in the blast, died on Sunday, editors of the English translation of his book said. He was 88, according to the Associated Press.

Mori was 8 years old when he survived the Aug. 6, 1945 U.S. attack on Hiroshima, an area described as roughly 2½ kilometers (1½ miles) from the blast. The attack instantly destroyed much of the city and killed tens of thousands, and by the end of 1945 the death toll reached 140,000, with a second atomic bomb later dropped on Nagasaki killing another 70,000, the AP reported.

About 30 years after the bombing, Mori learned that some American prisoners of war held in Japan were among those killed by the atomic bomb dropped by the United States. Working as a full-time company employee, Mori researched U.S. and Japanese official documents and tracked down 12 American POWs, AP said.

Mori then wrote letters to the bereaved families of those prisoners in the United States—families who, AP reported, did not know how their loved ones had died. His research, he later said, was not aimed at people from the “enemy country,” describing it instead as a project “about human beings.”

Mori authored the Japanese-language book “The Secret of the American POWs Killed by the Atomic Bomb,” which was published in 2008 and won him a Kikuchi Kan Prize, according to AP. The book was later translated into English, and the editors of that English edition said Mori died Sunday.

AP also reported that Japanese media said Mori died at a Hiroshima hospital. The AP said Mori’s research eventually led to U.S. confirmation of the deaths of the 12 captured American service members in the bombing.

Mori’s story gained prominent recognition during former President Barack Obama’s 2016 visit to Hiroshima’s Peace Memorial Park. In a speech there, Obama mentioned “a dozen Americans held prisoner” among the victims. The AP reported that Obama recognized Mori for seeking out those Americans’ families and later gave him a hug.

“The research I spent more than 40 years was not about people from the enemy country,” Mori said, according to AP. “It was about human beings,” a framing that helped explain how his historical documentation work turned into a personal effort to connect families across the Pacific with what happened in 1945.