TOKYO (AP) -- They drift into seas near Japan by the dozens every year, ghostly wrecked ships thought to come from impoverished North Korea.

Japanese authorities said Tuesday they are investigating nearly a dozen wooden boats carrying decomposing bodies that were found off the country's northwestern coast over the past month.

In most cases, the bodies are in such bad shape after being at sea for weeks that it's been impossible to determine their cause of death, officials say.

On Nov. 20, officials found 10 bodies in three boats off the coast of Ishikawa prefecture. Two days later, another wooden boat was found off nearby Fukui prefecture with six skulls, one nearly intact body with a head, and various other bones and remains, coast guard official Yuka Amao told The Associated Press. 

Coast guard officials said at least 11 shoddy fishing boats carrying the bodies have arrived since late October. Most are carrying equipment, nets and signs written in Korean, including one carrying a sign saying ``Korean People's Army,'' the North Korean army.

The officials said they could not say for certain, but the poor condition and small size of the 10- to 12-meter-long  vessels are not typical of South Korea or Japan, said coast guard spokesman Yoshiaki Hiroto. He said evidence suggests the boats are from the Korean Peninsula, though he declined to identify the country.

The recent spate of arrivals has drawn attention, though such discoveries are not out of the ordinary: dozens of such wrecked boats drift toward the Japan every year.

So far this year, 34 mystery boats have drifted over, including the 11 found between late October and November. Last year, Japan found 65 of them. In 2013, there were 80, according to the coast guard.

The number tends to rise during the fall and winter season because of prevailing winds from the northwest, Hiroto said.

In recent years, fishermen from food-short North Korea have increasingly forayed into Japanese waters hunting squid, and some of the boats found adrift have been carrying squid-catching equipment. They are usually ordered away when caught by the Japanese coast guard since the two countries lack a fishing agreement.

Sometimes disabled North Korean vessels are rescued with their crews intact, and repatriated. In other cases, fishermen have used their ships to defect to South Korea.

The history of such drifting arrivals goes back centuries, according to historical accounts.