Chief Air France spokesman Francois Brousse said "it is possible" that the plane was hit by lightning, as Brazil began a search mission off its northeastern coast.
"It is probably a combination of circumstances that could have led to the crash," he said, reported Reuters.
French Environment Minister Jean-Louis Borloo said hijacking had been ruled out as a possibility, according to the Agence France-Presse.
Air France Flight 447, an Airbus A330, left Rio at 7 p.m. local time Sunday with 216 passengers and 12 crew members on board, company spokeswoman Brigitte Barrand. The flight had been expected to land at Paris's Roissy Charles de Gaulle International Airport on Monday at 11:15 a.m.
Air France-KLM CEO Pierre-Henri Gourgeon, at a news conference at Charles de Gaulle Airport north of Paris, said the pilot had 11,000 hours of flying experience, including 1,700 hours flying this aircraft.
About four hours later, the plane sent an automatic signal indicating "electrical circuit malfunction" while going through strong turbulence, Air France said.
Brazil's Air Force said the last contact it had with the Air France jet was at 9:30 p.m. Sunday, but did not say where the plane was at that time.
The head of investigation and accident prevention for Brazil's Civil Aeronautics Agency, Douglas Ferreira Machado, told Brazil's Globo TV that he believes the plane must have left Brazilian waters and could have been near the coast of Africa by the time contact was lost, based on the speed it was traveling, according to the Associated Press.
"It's going to take a long time to carry out this search," he said. "It could be a long, sad story. The black box will be at the bottom of the sea."
Airport authorities have set up a crisis cell at Charles de Gaulle.
About 20 relatives of passengers on board the flight arrived at Rio's Galeao airport on Monday morning seeking information.
Bernardo Souza, who said his brother and sister-in-law were on the flight, complained he had received no details from Air France.
"I had to come to the airport but when I arrived I just found an empty balcony." he said. "With a lack of information, it is even more worrying."