Police accuse a mystery US couple of encouraging John Chau to go to North Sentinel Island as reports of "missionary boot camp" emerge.

Indian investigators believe two American missionaries encouraged John Allen Chau to go to a forbidden island where he was killed by an isolated, Stone Age tribe he was trying to convert to Christianity.

The Sentinelese are one of the world's last
The Sentinelese are one of the world's last "uncontacted" tribes whose language and customs remain a mystery to outsiders. Photo / Supplied

Andaman and Nicobar Islands police director-general Dependra Pathak told AFP the suspects had left India, and that there was still no sign of Mr Chau's body on North Sentinel Island.

A rare close up of a Sentinelese tribeswoman taken from 1991 footage of a visit by Indian anthropologists. Photo / Supplied
A rare close up of a Sentinelese tribeswoman taken from 1991 footage of a visit by Indian anthropologists. Photo / Supplied

The 26-year-old adventurer and missionary was killed last month in a hail of arrows fired by the Sentinelese tribe following a series of uninvited visits to the island, which is protected by the Indian Government.

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"We are investigating the role of at least two Americans, a man and a woman, who met with the man who went to the island," Mr Pathak said.

John Allen Chau attended a bizarre three-week 'missionary boot camp' in which he was mock attacked by people carrying fake spears and speaking gibberish. Photo / Supplied
John Allen Chau attended a bizarre three-week 'missionary boot camp' in which he was mock attacked by people carrying fake spears and speaking gibberish. Photo / Supplied

"These other two, who have since left the country, were reportedly into evangelical activities and encouraged him to visit the island."

The police chief did not name the couple or the organisation they belonged to but Mr Chau has been linked to Christian missionary group All Nations.

Chief Pathak said investigators had traced the American pair through calls made to Mr Chau's phone. The Americans had "local mobile numbers", he said.

Rare footage shows isolated Sentinelese Tribe reacting to outsiders. Photo / Survival International
Rare footage shows isolated Sentinelese Tribe reacting to outsiders. Photo / Survival International