The young American missionary killed by tribesmen on a remote island harbored an intense obsession with the secluded people — and “lost his mind” shortly before he was murdered, a friend said.

Remco Snoeij first met John Allen Chau, 26, in 2016 when the Washington state missionary showed up at his dive shop on Havelock Island, in India’s Andaman and Nicobar island chain, and said he wanted to learn to scuba dive.

Snoeij recalled that Chau seemed to be intensely interested in the North Sentinelese tribe, an isolated people living on the North Sentinel island, which is off-limits to visitors.

“He shared a keen interest in researching and knowing more about them,” Snoeij told the Washington Post.

On dive excursions, Snoeij told Chau about two fishermen who breached the island in 2006 and were strangled by islanders and about rumors that the Japanese military had buried gold there during World War II.

“It must have struck a chord” with Chau, he said.

Earlier in 2016, Chau had joined the All Nations missionary group. Mary Ho, the group’s international executive leader, told the paper that Chau had a “radical call” to find “unreached groups.”

“You could see that every decision he has made, every step he has taken since then was driven by his desire to be among the North Sentinelese people,” Ho said. He planned to live there for years and wanted to learn their language.

Since childhood, Chau seemed to have an interest in expeditions and explorers with his love for books like “Robinson Crusoe” and “The Sign of the Beaver,” about a boy left alone to guard his family’s log cabin with the help of his Native American friend.

After graduating from Oral Roberts University in 2014 with a degree in sports medicine, Chau lived in a cabin in the Whiskeytown National Recreation Area in California for three summers. At one point, he was sent to the hospital for a rattlesnake bite, the Post reported.

In 2016, Chau was already planning his mission to the North Sentinel island, telling a friend, John Middleton Ramsey, 22, that he was avoiding becoming romantically involved ahead of the trip.

“He knew of the dangers of this place,” Ramsey said. “He didn’t want any hearts to get broken should something go wrong. He was very much aware of what he was doing. He also knew it wasn’t exactly legal.”

On Nov. 14, Chau paid fishermen to take him to the island by boat at night. According to his diary, he saw men armed with bows and arrows and yelled, “My name is John, I love you and Jesus loves you.”

He returned the next day and the tribe fired arrows at him, piercing his waterproof Bible. On Nov. 16, he asked the fishermen to drop him off on the island, where he was killed by arrows, his body dragged and buried in the sand.

Indian authorities have now abandoned the dangerous mission to retrieve Chau’s body, so as not to disturb the Sentinelese, The Guardian UK reported.

Chau’s friends are still shocked by the incident.

“He lost his mind, definitely,” Snoeij said. “But ask any adventurer. You have to lose your mind a little bit, otherwise you don’t do it.”