An American man who was killed by tribespeople on a remote Indian island said he was travelling there to “declare Jesus to these people”.

In a letter to his family in the days before his death, missionary and adventurer John Allen Chau said they “might think I’m crazy”, but that he was willing to risk death to try and convert the people of North Sentinel island to Christianity.

It is illegal for anyone to travel to the small island, part of the Andaman and Nicobar chain which sits in the Bay of Bengal and is administrated by India. Members of the Sentinelese tribe have resisted contact with the outside world for generations and are known to attack outsiders with bows and arrows.

Mr Chau chartered a fishing boat to take him to the island and made a first, abortive attempt to contact the tribe on 15 November, according to the final excerpts from his journals that have been shared with US media.

Initial reports of Mr Chau’s death described him as an adventure tourist, and police told journalists it was “a case of misdirected adventure”.

But in his letter and journals Mr Chau makes clear that his primary motivation for visiting North Sentinel was that of a Christian missionary.

Describing his first encounter with the tribe Mr Chau wrote that he kayaked to shore and was met by a group of young people carrying bows and arrows.

“I hollered, ‘My name is John, I love you and Jesus loves you,’” he wrote. One of the young Sentinelese shot at him and the arrow pierced a copy of the Bible he was carrying.

The 27-year-old retreated and swam back to the waiting fishing boat, and that night wrote about his adventures and aspirations, leaving the journals and a letter for his family with the fishermen.

He returned to the island the next day, 16 November. On the morning of 17 November, the fishermen watched from the boat as members of the tribe dragged Mr Chau’s body along the beach and buried him, police said.

Now it is unclear whether the authorities will risk further conflict with the Sentinelese by travelling to the island to recover Mr Chau’s remains.

Sophie Grig, a senior official with the advocacy group Survival International who has been researching the Sentinelese people for more than two decades, said police were “still trying to work out” what to do about the body.

She noted that last time the Sentinelese killed outsiders – two Indian fishermen whose boat washed up on the shore of North Sentinel – attempts to land a helicopter on the island and bring back the remains were abandoned when the tribespeople fired on the aircraft with arrows.

She told The Independent: “In that case, the families said they appreciated the fishermen shouldn’t have been there, they knew [the laws against it] and we don’t think any more lives should be risked attempting to retrieve the bodies.”

John Allen Chau (right) with founder of Ubuntu Football Academy Casey Prince in Cape Town, days before he left for the Andamans (AP)

Dependera Pathak, director-general of police on India’s Andaman and Nicobar Islands, said Mr Chau was apparently shot and killed by arrows, but the cause of death could only be confirmed if his body is recovered.

Officials admitted they were exploring their options, since the law prevents them going directly to the restricted area and confronting the tribe. Police surveyed the island by air on Tuesday, and a team of police and forest department officials used a coast guard boat to travel there Wednesday and another trip was planned Thursday.

“We have to see what is possible, taking utmost care of the sensitivity of the group and the legal requirements,” said Mr Pathak.

Seven people have been arrested for helping the American reach North Sentinel island, including five fishermen, a friend of Mr Chau’s and a local tourist guide.

In an Instagram post, his family said it was mourning him as a “beloved son, brother, uncle and best friend to us”.

The family also said it forgave his killers and called for the release of those who assisted him in his quest to reach the island. “He ventured out on his own free will and his local contacts need not be persecuted for his own actions,” the family said.

In Mr Chau’s journals from his final days at North Sentinel, according to the Washington Post, he makes clear just how aware he was of the potentially deadly outcome of visiting the Sentinelese.

“I don’t want to die!” he wrote. “Would it be wiser to leave and let someone else continue? No. I don’t think so. I still could make it back to the US somehow as it almost seems like certain death to stay here.

“Why does this beautiful place have to have so much death here?” he wrote elsewhere. “I hope this isn’t one of my last notes but if it is ‘to God be the Glory’.”

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His letter to his parents begins: “You guys might think I’m crazy in all this but I think it’s worth it to declare Jesus to these people.

“Please do not be angry at them or at God if I get killed.” He signs it off “Soli Deo gloria” – “glory to God alone”.

Mr Chau had wanted ever since high school to go to North Sentinel to share Jesus with the indigenous people, said Mat Staver, founder and chairman of Covenant Journey, an organisation that takes college students on a tour of Israel to affirm their Christian faith. Mr Chau went through that programme in 2015.

He told the Associated Press: “He didn’t go there for just adventure. I have no question it was to bring the gospel of Jesus to them.”

Survival International estimates that there are around 100 tribespeople living on North Sentinel. They are primarily hunter-gatherers, although they also fish using small canoes, and unlike most of the protected tribes in India have been almost completely isolated from the outside world “for thousands of years, as far as we know”, Ms Grig said. Researchers don’t even know what language they speak, let alone what belief system they follow.

“People in the Andamans are appalled anyone would attempt to go there,” she said. “Not only is there an enormous risk to [the visitor’s] life, but it is potentially devastating for the tribe, who will have no resistance to the sorts of diseases you might bring in from outside. We know of Amerindian tribes where 90 per cent have been wiped out on first contact.”

A disastrous British colonial expedition to the island in 1880 could explain why the tribe is so hostile to outsiders. The explorers kidnapped two elderly Sentinelese tribespeople and four children, but when the adults quickly became sick and died, the children were immediately returned to the island with “gifts”.