By Fredrik Dahl

VIENNA (Reuters) - Libya's Gaddafi-era prime minister and oil chief Shokri Ghanem died after suffering heart failure and falling into the Danube river and there is no sign of foul play, the Vienna prosecutor's office said on Wednesday.

The mysterious death in late April shocked Ghanem's friends and colleagues, who at the time said they suspected enemies may have hunted down and killed the man who knew more than anyone else about the toppled Libyan dictator's billions.

But a spokesman for the prosecutor's office in the Austrian capital said he "definitely" did not see any suspicion that the 69-year-old had been murdered.

"There is no hint, no clue, that anything happened before he fell into the water," spokesman Thomas Vecsey said referring to an examination by experts of Vienna university.

Ghanem's body was found floating a few hundred meters from his home, fully clothed, near a promenade lined with bars and restaurants, where Viennese gather in the summer to sunbathe and drink beer. Police said he had been in the water a few hours, since about dawn on April 29.

There is no rail along the water's edge in that area, and it was not the first time a dead body had been found floating there.

Vecsey said the experts had concluded that Ghanem had very likely suffered heart failure and then fell into the river.

"He died of the heart attack but at the same time swallowed water," he said.

"The clothes were intact so there was no fight before, nothing that could us lead us to the thought that there was somebody else involved," he said, adding blood tests had only revealed normal levels of caffeine and nicotine.

Algae found in the corpse showed that Ghanem gasped twice for air before drowning, Austria's Kurier newspaper said.

Ghanem was one of the most powerful men in Gaddafi's Libya, effectively controlling the purse strings of the government and the Gaddafi family, until he defected to the opposition in May last year as rebels bore down on Tripoli.

His decision to switch sides was a turning point in the uprising that eventually drove Gaddafi from power. The former Libyan leader was later caught and killed by rebels.

Ghanem moved to a comfortable exile in Vienna, where OPEC - an organization of 12 oil-producing nations - has its headquarters and where two of his daughters lived with their families. He was still closely associated with Gaddafi's rule by Libya's new leaders and had ruled out returning home.

Vecsey said the investigation would continue into events before his death and to establish "why he went there, what he did there, what happened before, with whom was he in contact ... when this is finished we can finish the case."

(Additional reporting by Georgina Prodhan; Editing by Myra MacDonald)