(Ricardo Moraes ~ Associated Press)
Military aircraft scrambled out to the center of the Atlantic, far from the coasts of Brazil and West Africa, and France sought U.S. satellite help to find the wreckage. The first military ship wasn't expected to reach the area where the plane disappeared until Wednesday.
If there are no survivors, it would be the world's worst aviation disaster since 2001.
The official Agencia Brasil news agency Monday quoted Brazilian Air Force spokesman Col. Jorge Amaral as saying that a commercial airplane pilot saw what appeared to be fire on the ocean near the route taken by the Air France plane.
"There is information that the pilot of a TAM aircraft saw several orange points on the ocean while flying over the region ... where the Air France plane disappeared," Amaral said, referring to the Brazilian airline TAM. "After arriving in Brazil, the pilot found out about the disappearance and said that he thought those points on the ocean were fire."
French President Nicolas Sarkozy said the cause remains unclear and that "no hypothesis" is being excluded. Some experts dismissed speculation that lightning might have brought the plane down. But violent thunderheads reaching more than 50,000 feet high can pound planes with hail and high winds, causing structural damage if pilots can't maneuver around them.
Sarkozy said he told family members of passengers on Air France Flight 447 that prospects of finding survivors are "very small."
Brazil's president, Luiz Inacio Lula da Silva, expressed hope that "the worst hasn't happened," and said "we have to ask God" to help find survivors.
The 4-year-old Airbus A330 left Rio on Sunday night with 216 passengers and 12 crew members on board, said company spokeswoman Brigitte Barrand. Most of the passengers were Brazilian and French, but 32 nations in all were represented, including two Americans.
The plane was cruising normally at 35,000 feet and 522 mph just before it disappeared nearly four hours into the flight. No trouble was reported as the plane left radar contact, beyond Brazil's Fernando de Noronha archipelago.
But just north of the equator, a line of towering thunderstorms loomed. Bands of extremely turbulent weather stretched across the Atlantic toward Africa, as they often do in the area this time of year.
The plane "crossed through a thunderous zone with strong turbulence," Air France said. About 14 minutes later, an automatic message was sent reporting electrical system failure and a loss of cabin pressure. Air France said the message was the last it heard from Flight 447.
While what happened to the plane has not been determined, a Pentagon official said he'd seen no indication of terrorism or foul play. The official spoke on condition of anonymity due to the sensitive nature of the subject.
Chief Air France spokesman Francois Brousse said a lightning strike could have damaged the plane. Henry Margusity, a senior meteorologist for AccuWeather.com, noted that the thunderstorms towered up to 50,000 feet in the area, so it was possible that the plane flew directly into the most charged part of the storm.
Other experts doubted a bolt of lightning would be enough to bring the jet down. Some pointed to turbulence as a more dangerous factor.
"Lightning issues have been considered since the beginning of aviation. They were far more prevalent when aircraft operated at low altitudes. They are less common now since it's easier to avoid thunderstorms," said Bill Voss, president and CEO of Flight Safety Foundation, Alexandria, Va.
Voss said planes are built to dissipate electricity along the aircraft's skin, and are tested for resistance to big electromagnetic shocks.
The plane disappeared in an area of the mid-Atlantic ocean not covered by radar. Brazilian, African, Spanish and French air traffic controllers tried in vain to establish contact. The plane was gone.
Within two hours, two Brazilian Air Force planes began a search mission that grew Monday to seven aircraft and three navy ships. But with nothing more to go on than the last point where Flight 447 made contact -- about 745 miles (1,200 kilometers) northeast of the coastal city of Natal -- they faced an immense area of open ocean, with depths as much as 15,000 feet.
A French search plane took off from a military base in Senegal on Monday, to be joined by two more from France, and the Navy was asked to send a craft to help as well, armed forces spokesman Cmdr. Christophe Prazuck said.
Asking for U.S. satellite help, Sarkozy said finding the plane "will be very difficult."
"(I met with) a mother who lost her son, a fiancee who lost her future husband. I told them the truth," he said at a grim news conference in Paris.
The 216 passengers included 126 men, 82 women, 7 children and a baby, Air France said. There were 61 French and 58 Brazilians; 30 other countries were represented, including two Americans.
In Brazil, sobbing relatives were flown to Rio de Janeiro, where Air France was assisting the families.
At the Charles de Gaulle airport north of Paris, family members declined to speak to reporters and were brought to a cordoned-off crisis center.
Some people just missed disaster. Bernardo Ciriaco said there were two Air France flights leaving Rio for Paris Sunday night -- and his brother was on one of them. It was not until hours later that his brother, Gustavo, called from Paris to say that he had been bumped to the missing flight, but then talked his way onto the other one.
"Thank God he complained until he got back on the original flight. Our family is so relieved," Ciriaco said.
Air France said it expressed "its sincere condolences to the families and loved ones of the passengers and crew members" aboard Flight 447.
Air France-KLM CEO Pierre-Henri Gourgeon said the pilot had 11,000 hours of flying experience, including 1,700 hours flying this aircraft.
Experts said the absence of a mayday call meant something happened very quickly.
"The conclusion to be drawn is that something catastrophic happened on board that has caused this airplane to ditch in a controlled or an uncontrolled fashion," Jane's Aviation analyst Chris Yates told The Associated Press. "Potentially it went down very quickly and so quickly that the pilot on board didn't have a chance to make that emergency call."
If all 228 people were killed, it would be the deadliest commercial airline disaster since Nov. 12, 2001, when an American Airlines jetliner crashed in the New York City borough of Queens during a flight to the Dominican Republic, killing 265 people.
Airbus spokeswoman Maggie Bergsma said it was the first fatal accident of a A330-200 since a test flight in 1994 went wrong, killing seven people in Toulouse.
The Airbus A330-200 is a twin-engine, long-haul, medium-capacity passenger jet that can hold up to 253 passengers. There are 341 in use worldwide, flying up to 7,760 miles (12,500 kilometers) a trip.
Keller reported from Charles de Gaulle airport in Roissy, France. Associated Press reporters Emma Vandore, Laurent Lemel and Laurent Pirot in Paris; Alan Clendenning and Tales Azzoni in Sao Paulo and Marco Sibaja in Brasilia; Slobodan Lekic in Brussels, Belgium; Barry Hatton in Lisbon and Airlines and Transportation Editor Greg Stec in New York also contributed to this report.