There was no word from the pilots, no sign that anything was wrong with Air France Flight 447 as it streaked over the dark waters of the Atlantic on its way to Paris. And then it was gone.
All it left behind were automated pings signaling that something had gone wrong. The plane had been battling through ferocious thunderstorms. But what caused one of the world's safest commercial jets, with 228 passengers and crew, to simply vanish over a vast expanse of ocean may never be known.
"It's like going into a black hole," said Robert Ditchey, a Marina del Rey aviation consultant and co-founder of America West, now part of US Airways. "The airplane is pretty much on its own. It's hours away from help."
The disappearance of the Airbus A330 has fueled speculation that lightning, faulty electronics or pilot error may have brought the plane down. A navigation device used on the same type of plane recently malfunctioned on two flights, causing one to lose control.
The investigation is likely to be especially complex, made more vexing by the possible difficulties of finding wreckage that has either sunk to the bottom of the sea or dispersed over hundreds of miles of water.
The plane was probably traveling about 500 mph, and the pilots were checking in with traffic control about every half-hour, meaning the search area is likely to include hundreds of square miles of open ocean.
According to European investigators, the last voice communication from Flight 447 came when it was near Fernando de Noronha island, about 200 miles off Brazil's coast. The missing airliner was about to enter Senegal's air traffic control space when it vanished.
On Monday night, search aircraft looked for signs of the plane in the Atlantic about halfway between the Brazilian and African coasts.
Brazilian Vice President Jose Alencar said he had received information that the pilot of another airliner had seen glowing spots, possibly fire, in the sea more or less at the time the Air France plane disappeared.
Pilots flying a commercial jet from Paris to Rio de Janeiro for Brazil's largest airline, TAM, spotted what they thought was fire in the ocean along the Air France jet's route early Monday, the airline said in a statement e-mailed to the Associated Press.
Brazilian air force spokesman Col. Jorge Amaral said authorities were investigating the report, according to the Agencia Brasil official news service.
Investigators have been particularly baffled by the lack of any sign of distress or communication from the pilots before the plane vanished.
"This was so catastrophic, no signals, no nothing. Then bang, it is gone," said Robert E. Breiling, an air safety expert in Boca Raton, Fla.
Breiling said that in many catastrophic airliner disasters, events unfold too quickly for pilots to send messages. That was the case in the 1996 explosion of TWA Flight 800, he said.
Airline officials said the Air France plane transmitted numerous automated messages of system failures. Officials on Monday were unable to determine whether any of the failures caused the plane to crash.
It's not the first time that a passenger jetliner has vanished over the ocean, leaving investigators few clues. In many cases, extraordinary detective work narrows down what happened and the cases are solved. But in 1962, a Lockheed plane en route to Vietnam plunged into the Pacific Ocean with 107 people aboard. Investigators never found the plane or determined what caused the crash.
Such catastrophic events are unusual, Breiling said. According to federal data, only 6% of commercial airliner accidents occur during the cruise phase of the flight. Accidents during landing and approaches account for 65% of the crashes.
One of the most urgent tasks for Air France investigators is to find the flight data recorder and the cockpit voice recorder, known as black boxes, which could be thousands of feet below the surface of the ocean.
Those devices have sonar beacons that can operate as deep as 20,000 feet, according to manufacturers.
The region of the accident is a particularly dangerous stretch for jetliners.
"This accident occurred in the Intertropical Convergence Zone, which is a breeding ground for thunderstorms and cyclones," said Thomas Anthony, director of USC's Aviation Safety and Security Program.
"If you look at satellite imagery, you can see lines and lines of thunderstorms sometimes that stretch for hundreds and hundreds of miles. You can see tops of thunderstorms in the 55,000-to-60,000-foot range."
Those storms produce violent turbulence, hail and lightning.
Initial speculation centered around the possibility of a lightning strike, but safety experts said planes are built to withstand such strikes. The metal in the fuselage and the wings conduct electricity and keep the lightning from damaging the plane, said Vladimir Rakov, professor of electrical and computer engineering at the University of Florida and an expert on lightning.