Police found the remains of at least five people in a wooden boat on Sado Island in Niigata Prefecture on Saturday, a Japan Coast Guard official said.
They are suspected to be North Koreans.
Police found the heads of two people as well as five bodies in the boat’s stern at around 9:30 a.m., Coast Guard official Kei Chinen said, adding the cause of death is under investigation.
The official could not immediately confirm whether the heads belonged to any of the bodies or were from two other people.
The wooden boat had letters and numbers written in Korean on the outside of the hull, he added.
A police officer spotted the vessel on Friday afternoon. Police waited until Saturday before boarding it due to unstable weather.
Saturday’s discovery marked the second time since last month that a wooden boat has washed up on Sado’s shores, Chinen said. At least 156 suspected North Korean fishing vessels have been found washed up in Japan or drifting in Japanese waters this year, according to the daily Yomiuri Shimbun.
Experts say some of the fishermen are traveling far out to sea to satisfy government mandates for bigger catches.
But their old and poorly equipped vessels are prone to mechanical and other problems, including running out of fuel, and there are few ways for them to call for rescue if they get into trouble.
Some of them wash up with dead crew members aboard. These vessels are called “ghosts ships” by local media.
Last year, 10 North Koreans rescued from a tiny wooden boat drifting off Tohoku were deported. In October, around 60 members of a North Korean boat were rescued in the Sea of Japan after it sank following a collision with a Japanese patrol vessel.