NEW DELHI — John Allen Chau seemed to know that what he was about to do was extremely dangerous.

Mr. Chau, an American thought to be in his 20s, was floating in a kayak off a remote island in the Andaman Sea. He was about to set foot on one of the most sealed-off parts of India, an island inhabited by a small, enigmatic and highly isolated tribe whose members have killed outsiders for simply stepping on their shore.

Fishermen warned him not to go. Few outsiders had ever been there. And Indian government regulations clearly prohibited any interaction with people on the island, called North Sentinel.

But Mr. Chau pushed ahead, setting off in his kayak, which he had packed with a Bible. After that, it is a bit of a mystery what happened.

But the police say one thing is clear: Mr. Chau did not survive.

On Wednesday, the Indian authorities said that Mr. Chau had been shot with bows and arrows by tribesmen when he got on shore and that his body was still on the island. Fishermen who helped take Mr. Chau to North Sentinel told the police that they had seen tribesmen dragging his body on the beach.