An American missionary believed he had been "called" to convert the protected tribe who shot him dead with arrows when he arrived on their remote Indian island.

John Allen Chau, 27, paid local fishermen to help him get to North Sentinel Island, one of the world's most isolated regions in India's Andaman islands, last week.

Chau took a boat ride with the fishermen before venturing alone in a canoe to where the indigenous people live cut off completely from the outside world, authorities said.

As soon as he set foot on the island, which is off-limits to visitors, Chau found himself facing a flurry of arrows - but he kept walking, the Daily Mail reports.

The tribe then tied a rope around his neck and dragging his body away, according to the fishermen who helped him get there.

Indian police said a murder case had been registered against "unknown tribesmen" and seven people - the fishermen who took him to the island - have been arrested in connection with the death.

But the Sentinelese who killed Chau can't be prosecuted as contact with them and several tribes on the islands is illegal in a bid to protect their indigenous way of life and shield them from diseases.

Now, a friend has revealed to DailyMail.com that Chau was "committed" to travelling to the remote island, deep in the Indian Ocean, to preach Christianity to the tribesmen and had been planning the trip for at least three years.

Neil MacLeod, of Stornaway, Scotland, said he met Chau on a flight from London to Phoenix, Arizona, in October 2015.

"I saw him reading some Christian literature and I'm a Christian and we started talking," he said.

The investigation began after police were contacted by the US consulate in the southern city of Chennai, which has been in touch with Chau's mother.

MacLeod, 47, said Chau told him he had recently returned from India and was trying to figure out how to travel to the remote North Sentinel Island in the Bay of Bengal.

"He mentioned that he wanted to go to these islands, the islands where he has now died," MacLeod said.

"I had heard of these islands and I know how dangerous they are, so I was surprised by that."

He added: "He recognised the dangers of travelling there, but I think he had a sense of call.

"This was something he was working on for three years. He was committed to going there. In his view, he was trying to help these people.

"There are islands that are nearby and he was making relationships and connections to help him get to the islands."

Since the flight, MacLeod said he had emailed back and forth with Chau, who he described as "magnetic" and "charming."

"He was a lovely character and wanted to help people. The thing that came across was what a delight it was to be in his company.

"He was such a warm and engaging and friendly kind of fellow. You might have an idea of what a missionary might be like, he was a million miles from that.

"I think he's a real loss. I'm just very sorry about what has happened."

MacLeod said Chau was based in Portland, Washington, and working as an EMT at the time. He said he had helped during major incidents, including in the wake of Hurricane Katrina in 2005.

"He was a free wheel so he would go to incidents, like major incidents around the world and look to help," he added.

The Sentinelese tribe has attacked almost everyone who has entered their territory. Pictured, the tribes people are seen wielding arrows on the island.

The North Sentinel island is out of bounds even to the Indian navy in a bid to protect its reclusive inhabitants who number only about 150.

"He had worked with FEMA when he went down to Katrina. He was working there. He's worked in some pretty rough places."

He also paid tribute to Chau on Twitter, writing: "He readily spoke of his calling to serve the Sentinelese. His calling came from a higher authority.

"He died a servant of the Lord. Saddened by his loss. He was a lovely guy," he wrote.

Others flocked to Chau's Instagram to pay tribute, with one friend writing: "You were a real explorer out to understand the boundaries of terrestrial human travels.

"Those boundaries are seemingly electric at first touch... and you were the first to touch."

Another added: "So proud of you John. You are a true hero and it was such an honor to know you. Thank you for your obedience and passion, it's inspiring."

A police source told Reuters that Chau was a preacher who had visited the nearby Andaman and Nicobar islands in the past.

Police have learned he had a strong desire to meet the Sentinelese and preach on the island, the source added.

Chau was a Christian missionary who wanted to interact with members of the Sentinelese tribe, according to International Christian Concern.

William Stark, ICC's regional manager, paid tribute to Chau and condemned his killing.

"We here at International Christian Concern are extremely concerned by the reports of an American missionary being murdered in India's Andaman and Nicobar Islands.

"Our thoughts and prayers go out to both John's family and friends.

"A full investigation must be launched in this murder and those responsible must be brought to justice."

Chau tried to reach the island on November 14 but couldn't make it, police sources said.

Police have learned he had a strong desire to meet the Sentinelese and preach on the island

He added: "India must take steps to counter the growing wave of intolerance and violence."

Authorities say Chau had made several trips to the Andaman islands recently before finally managing to make it to the remote stretch by offering money to local fishermen.

Chau hired a fishing dinghy and, aided by the fishermen, reached the vicinity of the island on November 16, before transferring to a canoe, the official said.

His body, spotted the following day by the fishermen on their return, has not yet been retrieved, the official added.

Police have launched an investigation, Deepak Yadav, a police official in the island chain in the Bay of Bengal, confirmed in a statement late on Tuesday.

The investigation began after police were contacted by the US consulate in the southern city of Chennai, which has been in touch with Chau's mother.

"He tried to reach the Sentinel island on November 14 but could not make it," police sources said.

"Two days later he went well prepared. He left the dingy midway and took a canoe all by himself to the island.

"He was attacked by arrows but he continued walking. The fishermen saw the tribals tying a rope around his neck and dragging his body.

"They were scared and fled but returned next morning to find his body on the sea shore."

Dependra Pathak, the Andaman Director General of Police, told The News Minute that Chau's body hasn't yet been retrieved.

The Sentinelese tribe has attacked almost everyone who has entered their territory. Pictured, the tribes people are seen wielding arrows on the island.

"His body has not yet been retrieved because we have to strategise keeping in mind the nuances and sensitivity of other cultures," he said.

"We are working on that, and are in contact with anthropologists and tribal welfare experts. We will figure out some strategy."

A spokeswoman for the US consulate in Chennai said: "We are aware of reports concerning a US citizen in the Andaman and Nicobar Islands.

"When a US citizen is missing, we work closely with local authorities as they carry out their search efforts," she added, but declined to provide further details over privacy concerns.

Chau's Instagram shows he spent much of his time travelling and he documented his exploits on a blog called The Rugged Trail.

His last post on Instagram was shared on November 2. Alongside a series of pictures taken in the jungle, he wrote: "Adventure awaits. So do leeches."

Chau also spoke about his passion for travel in an interview with The Outbound Collective in 2014.

When asked where was on his "must-do adventure list," he said: "Going back to the Andaman and Nicobar Islands in India is on the top - there's so much to see and do there!'"

He also revealed his personal motto was: "Make the most of every good opportunity today because you don't know what's going to happen tomorrow!

Speaking about his inspirations, he said: "Adventurers like John Muir, Bruce Olson, and David Livingston inspire me to go travel and explore, and I definitely get my inspiration for life from Jesus."

Meanwhile, Survival International, a London-based organization that has been campaigning to protect the indigenous tribes living in the Andamans, called Chau's death a "tragedy" that should 'never have been allowed to happen.'

Chau tried to reach the island on November 14 but couldn't make it, police sources said.

The group's international director, Stephen Corry, said: 'This tragedy should never have been allowed to happen. The Indian authorities should have been enforcing the protection of the Sentinelese and their island for the safety of both the tribe, and outsiders.

"Instead, a few months ago the authorities lifted one of the restrictions that had been protecting the Sentinelese tribe's island from foreign tourists, which sent exactly the wrong message, and may have contributed to this terrible event."

He added: "It's not impossible that the Sentinelese have just been infected by deadly pathogens to which they have no immunity, with the potential to wipe out the entire tribe.

"The Sentinelese have shown again and again that they want to be left alone, and their wishes should be respected."

Corry said the British colonial occupation of the Andaman Islands "decimated the tribes living there, wiping out thousands of tribes people, and only a fraction of the original population now survive."

"So the Sentinelese fear of outsiders is very understandable," he said.

"Uncontacted tribes must have their lands properly protected. They're the most vulnerable peoples on the planet. Whole populations are being wiped out by violence from outsiders who steal their land and resources, and by diseases like the flu and measles to which they have no resistance.

He returned two days later better prepared. He left a dinghy midway and took a canoe to the island alone.

"Tribes like the Sentinelese face catastrophe unless their land is protected. I hope this tragedy acts as a wake up call to the Indian authorities to avert another disaster and properly protect the lands of both the Sentinelese, and the other Andaman tribes, from further invaders."

Both endangered aboriginal Andaman tribes - the Jarawa and the Sentinelese - are hunter-gatherers and any contact with outsiders puts them at risk of contracting disease.

Tourists often bribe local authorities to spend a day out with members of the 400-strong Jarawa tribe.

But in contrast, the Sentinelese tribe shuns all contact with the outside world and are known to be hostile to any encroaches.

The North Sentinel island is out of bounds even to the Indian navy in a bid to protect its reclusive inhabitants who number only about 150.

The Sentinelese attracted international attention in the wake of the 2004 Asian tsunami, when a member of the tribe was photographed on a beach, firing arrows at a helicopter that was checking on their welfare.

In 2006, two fishermen, whose boat strayed onto the 60-sq-km (23-square-mile) 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, believed to be the last pre-Neolithic tribe in the world.