An almost 40-year mystery is solved Friday when authorities rule that an Australian prime minister who disappeared Dec. 17, 1967 had drowned and was not assassinated or taken by a Chinese submarine.

Harold Holt disappeared while swimming in the ocean in the southern state of Victoria after serving almost two years as prime minister.

His body was never found, sparking a wave of rumors and often crackpot theories about his fate, including that he was killed by American assassins, Reuters reports.

A book on 59-year-old Holt's disappearance even claims he had been a spy and was picked up by a Chinese submarine and taken to Beijing.

But Victorian state coroner Graeme Johnstone says in a formal ruling Friday that Holt had drowned in rough seas at Cheviot Beach, south of Melbourne.

"Simply put, Mr. Holt took an unnecessary risk and drowned in rough water off Cheviot Beach," Johnstone ruled.

Johnstone's finding came after inquiries with Australia's chief spy agency, the Australian Security Intelligence Organization (ASIO), and after reviewing evidence and witness statements from those at the beach when Holt disappeared.

Until 1985, the coroner was not allowed to hold inquests in cases when a body was not found. Johnstone's belated finding came as part of a wider investigation into 85 drownings in Victoria where bodies were not found.