The bodies of another four men were found recently on the Sea of Japan coast, police said Thursday, the latest in a series of cases in which bodies apparently from North Korean boats have washed ashore in northern Japan.

A local resident called police after discovering one of the bodies washed ashore in the city of Oga in Akita Prefecture around 8:50 a.m. on Thursday. The police found a second body about 300 meters away at around 10:30 a.m. The heads of the bodies had been reduced to skulls, the police said.

They also found a wooden boat washed ashore in the neighboring town of Mitane, several kilometers from where the bodies were found. Judging from the shape of the boat as well as a life vest bearing Hangul letters found in it, the police suspect it came from North Korea.

Police in Aomori Prefecture said, meanwhile, they had found the bodies of two people in the town of Fukaura on Sunday and Tuesday, and one or two months had elapsed since they died. Suspected North Korean boats have been found washed ashore recently in the area.

Earlier in the day, another wooden boat, 7 to 8 meters long, was found capsized around 7 a.m. on a beach in Oga, the Akita police said.

Also on Thursday in Niigata Prefecture, a fisherman found a wooden boat floating around 2 km off Sado Island and notified the Japan Coast Guard around 7 a.m.