The US missionary killed by tribespeople on a remote Indian island had been preparing for years ahead of the journey and may not have acted alone, it has emerged.

John Allen Chau died on 17 November while attempting to spread Christianity to the protected island of North Sentinel in the India-administrated Andaman and Nicobar chain.

Police have for now abandoned attempts to land on the island and either retrieve the 27-year-old’s body or investigate his murder, saying they do not want to disturb the fragile lifestyle of the uncontacted Sentinelese tribe.

In the aftermath of Mr Chau’s death, authorities arrested five fishermen and two local contacts of Mr Chau on suspicion of helping him reach the island. All travel to North Sentinel, which has been sheltered from extensive contact with the outside world for decades, is illegal under Indian law.

Now police say they are investigating the possibility that Mr Chau had a series of meetings with another two American citizens in the Andaman capital of Port Blair, right up to the day he was due to leave for the island.

The Hindustan Times quoted two police officials as saying that a 53-year-old woman from Tennessee and a 25-year-old man from Colorado arrived in India, travelled to the Andaman islands and met with Mr Chau from 5 to 10 November in what he described in his journals as a “safehouse”.

Police said the two Americans, who were not named in the report, left the islands on 10 November and Mr Chau was supposed to start his journey to North Sentinel the next day. He was ultimately delayed by a cyclone in the Bay of Bengal, pushing back the trip until the 16 November.

Authorities reportedly have details for the two Americans and are considering whether to contact them as part of the investigation into Mr Chau’s death. Police chief Dependra Pathak told the ​Hindustan Times that two US nationals had indeed visited Mr Chau, saying: “It looks like Chau wasn’t alone.

“We are still probing the case. I won’t comment on an ongoing investigation,” he added.

Separately, the boss of the missionary charity which sent Mr Chau to North Sentinel has defended the young man’s actions, and said he decided to make the journey alone because he was “always thinking of other people’s safety first”.

In an interview with Christianity Today, the All Nations’ group’s international executive leader Mary Ho said: “We encourage all our missionaries to go two by two at the very least, and there were several others who were willing to go with John Chau.

“At the very end, he personally decided to go alone. He knew that he was going into a risky situation.”

Ms Ho said Mr Chau first contacted All Nations two years ago, already intent on travelling to North Sentinel. “He came to us because he wanted to be trained and sent out [to North Sentinel]. He knew we had experts in anthropology, in missiology, in linguistics.

“This young man was intent on fully equipping himself… He was thorough and meticulous in his preparations.”

Efforts to recover Mr Chau’s body from North Sentinel have been suspended amid calls from international rights groups, who say the tribespeople have no immunity to common diseases and could be decimated by contact with the outside world.

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Asked about fears that the Sentinelese could have been harmed by Mr Chau’s actions, Ms Ho claimed such concerns were not as relevant at “a time right now when there is modern medicine, when there is antibiotics”.

Mr Chau had “attempted to get 13 types of immunisation” before travelling and, just before getting the boat to North Sentinel, “quarantined himself for many days as a preventative step”, she said.

“None of us really know the exact health condition of the North Sentinelese,” Ms Ho said. “We do not know what kind of health issues [they have]. And perhaps there is some modern medicine that would be helpful for them.”

In a statement on Monday urging the Indian authorities not to disturb the Sentinelese further in their attempts to investigate Mr Chau’s death, the rights group Survival International said it was likely the result of previous disastrous contacts with the outside world that the tribespeople “defend their island by force”.

“The Sentinelese … face being wiped out if any outside diseases are introduced,” director Stephen Corry said.