Japanese police found the remains of at least five people inside a wooden boat with Korean writing on its side after it washed ashore on the coast of one of Japan’s outlying islands.

Officials said the boat, suspected to be from North Korea, was discovered around 9:30 a.m. local time Saturday on Sado Island, which is off the coast of Japan’s northwestern prefecture of Niigata.

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Coast Guard official Kei Chinen told Reuters that police found the heads of two persons, as well as five bodies, though couldn’t confirm whether the heads belonged to the corpses or were from two other people.

Local media reported that the remains were “partially skeletonized,” suggesting the victims may have been at sea for a long time.

A police officer first spotted the badly damaged wooden boat on Friday afternoon, but officers waited until Saturday before entering it due to unstable weather, Reuters reported.

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The grisly finds come amid strained diplomatic ties between North Korea and Japan over the north’s nuclear arms program.

On Friday, Japanese public broadcaster NHK sent an incorrect news bulletin that reported North Korea had launched a missile that fell into waters east of the Japanese archipelago. It issued an apology, claiming it was a media training alert.