The man had intruded on an island that is home to an isolated tribe known to fire at outsiders with bows and arrows. Police have arrested seven fishermen for facilitating the US citizen's visit to the remote island.
A US citizen is believed to have been killed by an isolated arrow-wielding tribe in one of the islands of India's remote archipelago of Andaman and Nicobar.
The man, identified as John Allen Chau, had illegally ventured onto the North Sentinel Island, which is out of bounds for visitors and is home to the Sentinelese tribe.
Police have arrested seven fishermen for facilitating the 27-year-old American's visit to the island. Efforts are under way to recover the man's body.
"A murder case has been registered," a police officer told Reuters news agency.
The Sentinelese, among the world's last tribes untouched by modern civilization, are known to resist all contact with outsiders, often attacking anyone who comes near.
Local media say Chau was a missionary, who had visited the archipelago in the past.
It's not known yet if he planned to preach on the North Sentinel Island, which the Sentinelese have inhabited for thousands of years.
Authorities launched an investigation after police were contacted by the US consulate in the southern city of Chennai, which has been in touch with Chau's mother.
"We are aware of reports concerning a US citizen in the Andaman and Nicobar Islands," said Kathleen Hosie, a consulate spokeswoman.
"When a US citizen is missing, we work closely with local authorities as they carry out their search efforts," she added. Hosie declined to comment any further, citing privacy concerns.
The isolate tribe, believed to be one of the last pre-Neolihic tribes in the world, attracted global attention in the aftermath of the 2004 Indian Ocean tsunami when one of them was photographed firing arrows at an Indian Coast Guard helicopter.
In 2006, the Sentinelese killed two fishermen when their boat broke loose and strayed onto the island, which is about the size of Manhattan. Their bodies were never recovered.
Estimates say the Sentinelese number between just 40 and 150. Their population is believed to be declining.
Survival International, which works for the rights of tribal people, said the killing of the American should prompt Indian authorities to properly protect the lands of the Sentinelese and other Andaman tribes.
"The British colonial occupation of the Andaman Islands decimated the tribes living there, wiping out thousands of tribespeople, and only a fraction of the original population now survives. So the Sentinelese fear of outsiders is very understandable," Stephen Corry, the group's director, said in a statement.
ap/msh (AP, Reuters, AFP, dpa)