An unsettling mystery has washed up on Japan's shores.

Over the past two months, at least 12 wooden boats have been found adrift or on the coast, carrying chilling cargo -- the decaying bodies of 22 people, police and Japan's coast guard said.

All the bodies were "partially skeletonized" -- two were found without heads -- and one boat contained six skulls, the coast guard said. The first boat was found in October, then a series of boats were found in November.

Coast guard officials are trying to unravel the riddle of where these ghostly boats came from and what happened to those on board.

Their best guess so far is that the ships are from North Korea.