A police spokesman, Roman Hahslinger, said that Mr. Ghanem’s body was spotted by a passer-by early in the morning, floating in the river near his Vienna residence. It was not immediately clear how he died, and an autopsy was planned.

“There would be no signs of violence if someone pushed him in,” Mr. Hahslinger said. “But it’s also possible that he became ill and fell into the water.”

Mr. Ghanem, who was 69, was found dressed normally but without any identification other than a document bearing the name of the company he worked for, Mr. Hahslinger said; a colleague identified the body.

Mr. Ghanem was the Libyan oil minister, part of Colonel Qaddafi’s inner circle and one of its most seasoned international negotiators in May 2011 when he abruptly fled to Tunisia under murky circumstances as the insurgency was gaining momentum and Libya was growing more isolated. For a few days the Qaddafi government insisted that he was traveling on business.

While working under Colonel Qaddafi, Mr. Ghanem insisted that Libya bore no responsibility for the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, which killed 270 people, nor for the shooting death of a British police officer during a protest outside the Libyan Embassy in London in 1984, an episode that led to the severing of British-Libyan relations. Before his tenure as prime minister from 2003 to 2006, he lived and worked in Vienna as a senior OPEC official.