Police are investigating whether American adventurer John Allen Chau had extra help reaching a remote island where he is reported to have been have been killed by an endangered tribe.

Indian authorities said the 26-year-old, believed to be a mountaineer and Christian missionary, may have had help from more people than initially thought to make his illegal voyage. He was allegedly killed earlier this month by people of the Sentilese tribe who inhabit the North Sentinel Island in the Andaman and Nicobar island chain.

In a statement, Andaman police said they would further investigate “various aspects of the case, including the sequence of events, the sea route followed for North Sentinel Island, the location where the victim landed and the place of the incidence, the location where John Allen Chau was last seen.”

Initially, police believed Mr Chau paid seven local fishermen — all of whom are currently under arrest — to assist him in reaching the island where tourism and photography is illegal.

The fishermen told police that they later saw his body being dragged across a beach and buried in the sand, identifying it as Mr Chau by his clothes and silhouette.

Authorities must now juggle the complicated process of investigating the death of a foreign national while simultaneously protecting one of most vulnerable populations on the planet.

The Sentinelese, hunter-gatherers armed with primitive spears and bows and arrows, are considered to be the last pre-Neolithic tribe in the world and the most isolated such group.

North Sentinel, located 31 miles (50 kilometres) west of Port Blair — the capital of the island cluster — is protected by laws which bar even fishing within a 5-nautical mile radius of the island. Those guilty of breaking the law face jail of up to three years.

Police have reportedly already attempted a recce of the island on multiple occasions to gain a better understanding of what may have happened 17 November.

“I understand the emotional concern of the family,” Dependra Pathak, director general of police in the Andaman and Nicobar islands, told Reuters. “But we’ll be handling the entire issue keeping in mind the law.”

“We have to respect the utmost sensitivities in this case,“ he added.

The tribe, estimated to be only a few dozen in number, have for decades aggressively resisted contact with the outside world.

Anthropologists were briefly in contact with the tribe in the early 1990s, but their effort was abandoned due to fears that contact with the outside world could expose the tribe to pathogens and lead to their extinction.

Some people who said they knew Mr Chau described him in glowing terms on social media, while others criticised his visit to the island.

“John wasn’t reckless; he was incredibly and profoundly filled with purpose and calling,” Sarah Prince, who described Mr Chau as a “dear friend”, said in a post on Instagram.

“His ‘adventure’ to India was to bring the love of Jesus to the Sentinelese people. Ultimately it is what he gave his life for, and he was prepared to do it,” she added.

Mr Chau had been able to make contact with the tribe and return to the fishing boat two or three times, according to notes he left with the fishermen that are now in custody.

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Mr Pathak said authorities were looking into whether they could retrieve his body.

“What has happened is not very clearly spelled out in his notes, but it is indicative of his deep expedition planning and his determination to contact these aborigines despite knowing he would face vigorous rejection,” he said.

Mr Chau’s social media posts identify him as an adventurer and explorer. Responding to a travel blog query about what was on the top of his adventure list, he wrote: “Going back to the Andaman and Nicobar Islands in India.”

He also said in the blog: “I definitely get my inspiration for life from Jesus.”

Based on his social media posts, he appears to have visited India multiple times in the last few years, exploring and preaching in many parts of southern India.

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“We recently learned from an unconfirmed report that John Allen Chau was reported killed in India while reaching out to members of the Sentinelese Tribe in the Andaman Islands,” members of Mr Chau’s family said in a post on his Instagram page.

The family asked that local contacts not be prosecuted in the case, saying they have forgiven the tribe and others who may have helped him reach the island.

They described him as a “beloved son, brother and uncle” as well as a Christian missionary, wilderness emergency medical technician, soccer coach and mountaineer.

“He loved God, life, helping those in need and had nothing but love for the Sentinelese people,” the family said. “We forgive those reportedly responsible for his death. We also ask for the release of those friends he had in the Andaman Islands.”

In 2006, two fishermen who strayed onto the island were killed and their bodies never recovered. An Indian Coast Guard helicopter sent to retrieve the bodies was repelled by a volley of arrows from the community.