A “ghost boat” was discovered in Japan carrying five corpses and two human heads, according to a report.

The damaged boat, which was painted with Korean lettering, was beached Friday on Sado Island, where authorities unearthed the “partially skeletonized” remains, the BBC reported. 

It’s unclear if the decapitated heads belonged to the corpses.

Mangled fishing vessels with no one on board have become known as “ghost boats” in Japan. Most hail from Korea and hold the remains of fishermen who die from exposure and starvation during winter months.

Previous reports have speculated that the boats could belong to North Korean spies.