Japanese authorities are investigating nearly a dozen wooden boats carrying decomposing bodies found drifting off the northwestern coast over the past month.

Coast Guard officials said Tuesday they have found at least 11 shoddy boats carrying the bodies of unknown nationality since late October. They have also found fishing equipment and nets on board and signs written in Korean, suggesting they came from North Korea.

Officials say dozens of wrecked boats drift toward Japan's northwestern coasts every year.

On Nov. 20, officials found four bodies in two capsized boats off the coast of Ishikawa prefecture. Two days later, seven bodies were found inside another wooden boat off nearby Fukui prefecture.

Coast Guard said officials autopsied some of them but could not determine their nationality or cause of death.