RIO DE JANEIRO, Brazil - An Air France jet with 228 people on a flight to Paris vanished over the Atlantic Ocean after flying into towering thunderstorms and sending an automated message that the electrical system had failed.

A vast search began yesterday, but all aboard were feared dead.

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 tomorrow.

If there are no survivors, it would be the world's worst aviation disaster since 2001.

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 Sunday night, carrying 216 passengers and 12 crew members, said company spokeswoman Brigitte Barrand. Most of the passengers were Brazilian and French, but 32 nations in all were represented. Two Americans were on board.

The twin-engine jet 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, at 10:48 p.m. local time.

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, at 11:14 p.m. local time, (10:14 p.m. EDT Sunday), 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.

Chief Air France spokesman Francois Brousse said lightning could have damaged the plane. Henry Margusity, a senior meteorologist for AccuWeather.com, said it was possible that the plane flew directly into the most charged part of the storm because of the towering thunderstorms.

Other experts doubted a bolt of lightning would be enough to bring the jet down. Some pointed to turbulence as a more dangerous factor.

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.

Within two hours, two Brazilian Air Force planes began a search mission that grew yesterday to seven aircraft and three navy ships. But they faced an immense area of open ocean, with depths as much as 15,000 feet. *