Kazuki Takahashi, author of the popular Yu-Gi-Oh! manga series, has been found dead after an apparent snorkelling trip in Japan, a coast guard official said on Thursday.
The manga artist and game creator, 60, was found off the coast of Okinawa in the country's south on Wednesday.
He was wearing a T-shirt and an underwater mask, snorkel and fins, the official in the city of Nago, who declined to be named, said.
"We are investigating the case as both a possible accident or crime," he said.
Japanese media including public broadcaster NHK said Takahashi was found about 300 metres offshore and pulled from the water.
Yu-Gi-Oh! ran in the Japanese weekly comic magazine Shonen Jump between 1996 and 2004.
It told the story of Yugi, a teenager who receives an ancient puzzle that awakens in him the alter ego of an Egyptian pharaoh.
The manga gave rise to a media franchise including a trading card game as well as anime series and films.
It was later published as a series of 38 books by Japanese publishing company Shueisha, which did not issue any immediate statement on Takahashi's death.
Takahashi wrote under a pen name and his real first name was Kazuo.