TOKYO (AP) — Two boats carrying 21 people capsized Monday off Henoko, a controversial relocation site for a U.S. military base off Japan’s southern island of Okinawa, throwing all into the water and leaving two of them dead, officials said.

The Japan Coast Guard said 18 of them were students from a Kyoto high school on two boats, 10 on Heiwa Maru and eight on the smaller Fukutsu, to observe the Henoko area as part of their peace education program.

Coast guard rescuers pulled all 21 people out of the water, but a 17-year-old female student and the captain of Fukutsu were later pronounced dead, officials said. Two people were injured but their conditions are not life threatening.

Coast guard officials said the cause of the accident is under investigation.

The boats were about a kilometer (half a mile) east of Henoko when they capsized. A wave advisory was in place during the accident, but the water was not very rough and there was no sign the boats collided, officials said.

Persistent protests and lawsuits between Okinawa and Tokyo have held up the relocation plan of the U.S. Marine Corps Air Station Futenma from a crowded neighborhood on the island for nearly 30 years.

Henoko is a popular destination for activists opposing the relocation, but the students were not protesting, officials said.

Okinawa is home to about half of the 50,000 American troops stationed in Japan under the bilateral security pact. Many Okinawans complain about risks of accident, noise, pollution and crime associated with U.S. bases.