Authorities have not yet identified the origin of the boat, but said there were faint markings on the side of it that appeared to be Korean characters, CNN reported. But the characters were so faint that they could not be read clearly, said Toshiyuki Kojima, deputy chief of Sado Nishi Police Station.
The boat was discovered Wednesday morning, drifting into a rocky part of the coast at Mano Bay in Sado City, police said. Authorities did not know how long the boat had been there.
Police did not say how many bodies were found aboard the vessel, but local media reported five bodies were found.
Some of the bodies were decomposed to the point where only skeletons remained.