An American missionary who was killed by an uncontacted tribe last week meticulously planned to reach the island for years before his death, one of the man's friends says.

John Chau, 26, of Vancouver, Wash., first arrived in India’s Andaman and Nicobar Island chain in 2016 when he began taking diving lessons at Remco Snoeij’s shop, The Washington Post reported Tuesday. While his time at the dive shop didn't appear out of the ordinary, authorities believe Chau was scouting the island where the North Sentinelese roam and visited the area at least three times.

“He shared a keen interest in researching and knowing more about them,” Snoeij told The Washington Post. “It must have struck a chord.”

Chau made covert missions to the prohibited island and camouflaged “his expedition as fishing activity,” director-general of police for the Andaman and Nicobar Islands Dependra Pathak told the newspaper.

John Middletown Ramsey, 22, one of Chau’s friends, admitted to The Post that when Chau stayed with him in 2016, the island was on his mind and Chau even put off any attempt at a romantic relationship with anyone because of his desire to go to the island.

“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.”

Chau then joined the Missouri-based missionary group All Nations. He was described by one of the group’s leaders as having a desire to reach the North Sentinelese to learn their language, according to The Washington Post.

A diary provided to The Washington Post revealed Chau arrived in the Andamans on Oct. 16 and paid fishermen on Nov. 14 to take him to the island, where he interacted with the North Sentinelese people. Chau was later killed when a tribesman shot him with an arrow and later buried his body on the beach.

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

Chau’s death ignited a firestorm over whether to bring his body home. Fishermen apparently saw the Sentinelese bury Chau’s body on the beach. Authorities have said they didn’t want to disturb the islanders and human rights organizations have called any potential mission “incredibly dangerous.”

Scholars believe the Sentinelese are descendants of Africans who migrated to the area about 50,000 years ago and survive on the small, forested island by hunting, fishing and gathering wild plants. Almost nothing is known of their lives, except that they attack outsiders with spears or bows and arrows.

The Associated Press contributed to this report.