Two American missionaries are believed to have encouraged John Allen Chau to visit the Sentinelese tribe, Indian police said on Saturday.
There is still no sign of the body of Chau, who was killed last month by the Sentinelese tribe.
Dependra Pathak, head of police in the Andaman and Nicobar islands, told AFP the suspects had left India. "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," Pathak said.
"These other two, who have since left the country, were reportedly into evangelical activities and encouraged him to visit the island."
The identity of the suspects has not been revealed.
Indian police have so far arrested seven people, including six fishermen who helped Chau get to the island.
"So far, we have already done three reconnaissance trips near the island. We may yet go there again if needed to get a clearer picture of the sequence of events leading to the incident," said Pathak.
Police also believe that the Sentinelese may have exhumed Chau's body at the beach where he was killed.
"One of the reasons we went there was to try to spot the man's body, especially if the Islanders exhumed it," Pathak said. "Now it is about two weeks since the incident and it doesn't look likely that the American's body will be exhumed, wherever the islanders may have buried it."
