An autopsy done on Shukri Ghanem has found that the former Libyan oil minister drowned, Austrian police say.
Roman Hahslinger, police spokesman, said on Monday that the autopsy results on Ghanem's corpse also showed there were no signs of violence.
Ghanem's fully clothed body was found in the Danube in Vienna on Sunday, a few hundred meters from his home.
Hahslinger suggested the death may have been an accident and that Ghanem had complained to his daughter late on Saturday that he was not feeling well.
He said no suicide note has been found and there is no evidence Ghanem was under threat.
However, Vienna police and a Libyan security source suggested to the Reuters news agency that Ghanem, 69, could have been murdered.
The security source said they were investigating the death and believed he could have been pushed into the Danube by former Gaddafi agents.
Hahslinger said the results of toxicological tests were expected later this week as part of the investigation into the drowning.
"[Authorities] have to be very careful about what they say," Al Jazeera's Nadim Baba reported from Vienna on Monday. "But they have said that following the forensic autopsy, the preliminary results suggest that he drowned; they are not going any further than that.
"They are saying that they can't rule anything out but the police have told me that they consider it unlikely that it will emerge that there was foul play involved.
"They previously said that there were no signs of violence, although the police did indicate that that would be the case even if Shukri Ghanem was pushed into the Danube," our correspondent said.
Ghanem's body was found at 8:40am local time (06:40 GMT) on Sunday by a passerby near the entertainment area known as Copa Cagrana, where a footpath winds along the riverbank.
The former Muammar Gaddafi confidant, who was also close to Gaddafi's son Saif al-Islam, was privy to potentially damaging information, including on oil deals with Western governments.
Ghanem had been chairman of Libya's state-owned National Oil Corporation (NOC) before defecting last year
several months after opponents of Gaddafi had risen up against the Libyan leader and begun a rebellion.
He had been working of late as an energy consultant in Vienna, where two daughters and their families also live.
Ghanem served as prime minister under Gaddafi from 2003 to 2006 and then as oil minister until 2011.
He defected last June during the uprising that toppled Gaddafi.
Ghanem helped steer the country's oil policy and held the high-profile job of representing Libya at OPEC meetings.
At the height of the crisis in Libya, he crossed over to neighbouring Tunisia in mid-May 2011, by car and became one of the highest-ranking officials to defect from the regime.
His defection came just weeks before he was due to represent Libya at an OPEC meeting in Vienna.
In June, he announced his defection, saying he left his home country "to join the choice made by young Libyans to fight for a democratic country".
However, Ghanem said he was not working with the National Transitional Council, which is Libya's current interim government.
Instead, he sought refuge in Vienna, a city he knows well having not only travelled there regularly as oil minister for OPEC meetings, but also having lived there when he was director of OPEC's research division from 1993 to 2001.