TOKYO (AFP) - Japan's cyberspace lit up with talk of conspiracy on Wednesday when officials closed the case on the drowning at sea of a high-flying government economist who was apparently trying to smuggle himself home undetected from South Korea.

The lifeless body of Mr Kosuke Shiraishi, 30, was found in January off the coast of Kitakyushu in south-west Japan, along with the motorised rubber dinghy he had bought in South Korea days earlier.

Analysis of Mr Shiraishi's computer, left behind in a hotel in Seoul, has shown he was trying to reach a "certain destination in Japan for personal reasons", a spokesman from Japan's coast guard said, declining to give further details, citing privacy.

Mr Shiraishi, an official at the elite Cabinet Office and a specialist in macroeconomics, had been sent to study at a United States graduate school in July last year, according to earlier coast guard and press reports.

With the approval of his managers, he travelled to South Korea from the United States in January on the pretext of attending an international economics seminar, the reports said.

But he never showed up at the gathering and instead set off from the southern port of Busan, 400km away, apparently headed for Japan.

An autopsy on Mr Shiraishi's body detected no drugs or any injuries consistent with a struggle and there was no evidence he had been in contact with third persons in South Korea, the coast guard official said.

"We have determined that he accidently ingested water and drowned while drifting. There was no sign of foul play," the spokesman for the coast guard in the southern main island of Kyushu, told AFP by telephone.

But Internet users were not appeased by the explanation, with several speculating about the possible involvement of intelligence agencies or some other intrigue.

The refusal to divulge the contents of Mr Shiraishi's laptop was particularly inflaming.

"If these 'private' matters on this kind of a case are not made public, it will let conspiracy theories go on the loose," tweeted @Ki-46ozzie.

User @zapa wrote: "When you want to snuff someone out, put him/her on a rubber dinghy and let it capsize."

"When we talk about his death, several important cards seem to have been placed face down," said @sis-sis.