Indian authorities on Friday said they were investigating whether an American citizen killed by an isolated tribe had the help of more people than previously thought.

John Chau, a 26-year-old described as a fervent Christian, was killed last week when he illegally traveled to North Sentinel island, where an isolated pre-Neolithic tribe exists.

Members of the tribe shot arrows at Chau, killing him. Fishermen that aided Chau in his dangerous voyage said they saw tribesmen drag Chau's body across the shore and bury him in the sand.

'God sheltered me'

Police confirmed that excerpts from a diary recovered from one of the fishermen and published in local newspapers were indeed his words.

In his diary, Chau said he believed he would be killed if he returned to the island after making illegal contact days before. He had already sustained injuries from arrows, local media reported.

"God sheltered me and camouflaged me against the coast guard and the navy," Chau wrote shortly before he was killed, referring to regular patrols around the island to prevent outsiders from attempting contact with the Sentinelese tribe.

Read more: Civilization threat to isolated tribe's survival

'Love for the Sentinelese'

In the wake of Chau's death, police arrested a friend believed to have helped him in his journey along with several fishermen suspected of bringing him near the island.

Chau's family has called on Indian authorities to release their son's friend along with the fishermen, saying they should not be held responsible for his actions and they would not push for legal action against the tribe.

"He loved God, life, helping those in need, and had nothing but love for the Sentinelese people," the family said in a statement. "We forgive those reportedly responsible for his death."

Indian authorities have consulted anthropologists and tribal warfare experts to see if it would be possible to recover Chau's body. Dependra Pathak, police chief in the Andaman and Nicobar islands where North Sentinel is located, said law enforcement would continue to follow legal norms in the matter.

"I understand the emotional concern of the family," Pathak told Reuters news agency. "But we'll be handling the entire issue keeping in mind the law."

Read more: 'Tribal people have the right to determine how they wish to live'

Vulnerable people

The world is home to more than 100 uncontacted tribes, according to Survival International, a charity working on tribal people's rights. However, the tribes face enormous threats from contact.

"Whole populations of uncontacted tribes are being wiped out by genocidal violence from outsides who steal their land and resources, and by diseases like flu and measles to which they have no resistance," says Survival International.

Every evening, DW's editors send out a selection of the day's hard news and quality feature journalism. You can sign up to receive it directly here.

ls/msh (Reuters, AP)