When Indian police officers in a small boat pulled within sight of the remote island, they saw something strange. A group of islanders were huddled on the beach. Carrying bows, arrows and spears, they appeared to be guarding something.

Police officials said it could have been the body of John Allen Chau. The 26-year-old American missionary was killed last week as he tried to spread Christianity to North Sentinel, a forbidden island in the Andaman Sea with a long history of repelling outsiders.

The crew turned off the boat’s motor. They peered at the islanders through binoculars, making sure to stay several hundred yards off shore, out of bow-and-arrow range.

“The Sentinelese were watchful,” Dependra Pathak, the area’s police chief, said on Saturday. “They were patrolling the beach, at the same spot John was killed, with weapons.”