Japanese authorities have presented a gruesome mystery with 12 incidents of wrecked wooden boats appearing on their coastline since October, bearing a total of 22 decomposed bodies.

No official statement has been made on the corpse’s origins, but basic observation indicates they’re likely the remains of North Koreans. One boat had a handwritten sign identifying it as part of Unit 325 of the North Korean army and a scrap of cloth on the same boat fits the pattern and colors North Korea’s flag. The Korean People’s Army’s fishing boats have reportedly been under pressure to travel beyond what would be considered safe distances into the rough sea as Kim Jong Un demands larger catches and leans heavily on fisheries to boost the country’s notoriously low food supply.

Another theory speculates these are the remains of defectors now trying to cross the dangerous Sea of Japan because Chinese boarders are now more heavily policed. The vessels’ 33-foot to 40-foot length suggests they launched from the Korean Peninsula, as boats of that size are not common in South Korea or Japan.

Cause of death is harder to determine. The bodies decomposing at sea for weeks as they float toward Japan leave meager evidence for medical exams.

The bodies present a riddle, but a predictable one. Last year, the Japanese coast guard recovered 65 similar boats, and another 80 in 2013, with numbers spiking over fall and winter months as the winds blow in from the northwest.

Photos via Getty