The night sky was clear above the clouds, and the last glimmer of a setting half-moon had faded when Malaysia Airlines Flight 370, cruising at 35,000 feet over the Gulf of Thailand, approached the border between Malaysian and Vietnamese airspace on its usual route to Beijing. What happened next should have been routine for a twice-daily milk run between two of Asia’s most important cities. Air traffic controllers outside Kuala Lumpur usually hand the jet off to their counterparts in Ho Chi Minh City as the flight turns northeast toward the Chinese capital.

But in those early hours of March 8, pilots flying nearby heard an unusual crescendo of chatter on the radio frequencies used by radar control in Vietnam and Malaysia. Air traffic personnel in both countries were trying and failing to reach the plane.

"Any stations in contact with Malaysian 370, please relay."

Vietnamese and Malaysian controllers asked one aircraft after another to radio the jet. Pilots listened as one plane after another tried and heard only static.

"Malaysian 370, this is Malaysian 88."

"Malaysian 370, this is Malaysian 52."

People familiar with the calls, describing them for the first time, said they were calm, even laconic. The pilots trying to reach the airliner had no reason to believe it had suffered anything more than an ordinary radio malfunction. But those initial attempts to find a plane in the skies would soon evolve into an urgent multinational search operation spanning land and sea in two hemispheres. They signaled the start of what has become perhaps the most perplexing case in modern aviation – one that investigators say may take years to solve, or could remain a mystery forever.

More than two weeks after Flight 370 disappeared, unbridled speculation surrounds the unfolding global drama. So much is uncertain about what happened on the plane, and so much of what has been disclosed by Malaysian authorities has been contradicted, that hardly any theory of its fate can be easily dismissed. On Saturday, the authorities said a Chinese satellite had made a new sighting of a possible object floating in the southern Indian Ocean in the area that is now the focus of the search, and China was sending ships to investigate.

Based on dozens of interviews with people whose lives were touched by the plane as well as with outside experts and investigators from the two dozen countries searching for answers, this report presents a portrait of Flight 370 and the search to find it using what is known to date. But by necessity, it is an incomplete picture.

A ROUTINE NIGHT

Malaysia Airlines flies from Kuala Lumpur to Beijing twice daily, nonstop. The red-eye, departing at 12:35 a.m. and landing at 6:30 a.m., can carry a hodgepodge of travelers: vacationing Malaysian families, European businessmen pursuing deals, Chinese tourists returning home after visiting relatives or shopping.

Passengers are advised to arrive at least two hours in advance. On March 7, a delegation of 34 Chinese artists, relatives and organizers who had spent the past several days participating in an art exhibit in Kuala Lumpur played it safe with the city’s unpredictable Friday night traffic and arrived about 8 p.m.

Perhaps the most prominent of the artists was the flight’s oldest passenger, Liu Rusheng, 77, a calligrapher who had published an essay about how much he treasured life because he had "cheated death" six times, beginning when he was abandoned temporarily by his parents as they fled Japanese soldiers invading China.

Liu had "the energy of a young man," recalled Daniel Liau, the delegation’s host, who accompanied the travelers to the airport and helped them check their luggage. They stood chatting under the modernist scalloped ceilings of the main terminal for about 90 minutes. Later, after the group had passed through security and taken the monorail to the satellite terminal, Liau called them one last time.

"How are the artists?" he recalled asking one of the organizers, Hou Bo, who replied they had already reached the gate. "Everybody is OK," Hou assured him.

Boarding began about midnight. The airline would have allowed the elderly – including Liu and his wife, Bao Yuanhua, 73 – and the families traveling with the two infants booked on the flight to get settled first. Next came the passengers holding passes for the 35 seats in business class.

Philip Wood, 50, an IBM executive from Texas and a regular on the flight because he was relocating from Beijing to Kuala Lumpur, was in coach but hoped to be upgraded. He had long legs and a platinum frequent-flier card, said his partner, Sarah Bajc, 48, a teacher.

Bajc said she exchanged a dozen text messages with him before the flight about the movers, scheduled to arrive at their home in Beijing the next morning. "We discussed the state of packing, what still needed to be done," she said. His last message came just before he left for the airport.

Others on the flight were just passing through Kuala Lumpur, including Shi Xianwen, 26, a new father returning to China from a business trip to Australia. At the airport in Perth, he spent 40 minutes picking out a bracelet watch for his wife, whose birthday was approaching, an employee at the duty-free shop said.

And two passengers boarded using stolen passports: Pouria Nourmohammadi Mehrdad, 19, and Seyed Mohammed Reza Delavar, 29, Iranian men described by Interpol as migrants being smuggled into Europe.

Mohammad Mallaeibasir, 18, an information technology student in Kuala Lumpur, said the pair stayed in his apartment the night before they left. Mehrdad, a friend from high school in Tehran, he said, told him he was starting a new life in Hamburg, Germany, where his mother was waiting.

"He was quite nervous," Mallaeibasir recalled. "I could see it on his face."

The next night, he drove them to the airport and offered to help them check in, but they insisted on entering separately, Delavar first. The two high school buddies waited in the car for five to 10 minutes, smoking cigarettes, before Mehrdad got out to leave.

Mallaeibasir gave him a hug, told him to have a safe flight and watched as his friend carried a large backpack and a laptop computer bag into the terminal. That was the last he saw of him.

THE AIRPLANE

The plane the passengers boarded was a Boeing 777, one of the world’s most popular and advanced passenger jets, and Boeing’s first fly-by-wire commercial aircraft, in which electronic controls replaced manual ones. Pilots send commands that are conveyed to the wings and other components, and a computer helps keep the plane steady.

The "Triple Seven," as it often called, has all but replaced the 747 because it is cheaper to operate and can fly up to 16 hours without stopping to refuel. It also has one of the industry’s best safety records, with only two serious accidents in the 19 years it has been in service.

Malaysia Airlines, the nation’s state-run carrier, began using the Boeing 777 in 1997 and eventually had 15 in its fleet. One of them, the 404th model to roll off Boeing’s assembly line in Everett, Wash., was delivered to the airline in May 2002 and registered with tail number 9M-MRO. This was the plane used for Flight 370 that has disappeared.

Malaysia Airlines has said the jet has been involved in only one previous safety incident. On Aug. 9, 2012, the tip of one of its wings broke off after it clipped the tail of a China Eastern Airlines Airbus A340 while taxiing at Shanghai Pudong International Airport. No one was hurt. Boeing said it sent a team of engineers and mechanics to remove and replace the damaged wing sections, and returned the plane to service after testing.

Boeing recommends a light maintenance inspection of the 777 after about 500 hours of flight time. Known as an "A check," the inspection usually is conducted in a hangar by a team of about 15 engineers working about 10 hours, often overnight. Malaysia Airlines said the jet’s last "A check" took place Feb. 23, and uncovered no problems.

By the time it pulled up to the gate at Kuala Lumpur International Airport on March 8, the plane had completed more than 7,500 flights and clocked over 53,400 hours in the air, according to Flightglobal, a news and data service for the aviation sector. That put it well within the average economic life of 23 years for a wide-body passenger jet.

In other words, there was little to distinguish this plane from the roughly 1,170 other Boeing 777s now in use. That is why it is so crucial to determine if the plane’s disappearance was due to any malfunction or defect related to its design, build or engineering.

"The industry does not like uncertainty," said Mark Rosenker, a former chairman of the National Transportation Safety Board. "We will find out what happened."

BOARDING

As they stepped on to the plane, the 227 passengers of Flight 370 were greeted by the flight attendants, four women in sarong kebayas and six men in gray three-piece suits. Some distributed hand towels, juice and newspapers in business class; others helped those in economy find their seats.

Outside, ground crews loaded the passenger luggage into the jet’s cargo hold, which can carry up to six pallets and 14 shipping containers. The airline said no hazardous or valuable goods were on the flight. But among the cargo were a "significant" number of lithium batteries – which can be flammable – more than is typically sent in a shipment, one U.S. official said.

After the doors closed, the chief steward, Andrew Nari, would have welcomed the passengers via the loudspeaker and reminded them to turn off their cellphones. Before shutting off his own, he sent a message to his mother. "It was just a normal SMS telling me that his plane would fly off soon," she later told The Star, a local newspaper.

In the cockpit were the pilots: the captain, Zaharie Ahmad Shah, 53, who joined the airline in 1981 and had 18,365 hours of flying experience, and his first officer, Fariq Abdul Hamid, 27, who was transitioning to the Boeing 777 from the airline’s narrow-body fleet.

After nudging away from the gate, the plane taxied to runway 32R. The cabin lights would have dimmed before one of the pilots asked the crew to be seated for takeoff.

With two Rolls-Royce Trent engines, each capable of generating more than 92,000 pounds of thrust, the jet raced down the 2.5-mile-long runway and lifted off at 41 minutes after midnight. As the plane banked and climbed, some passengers might have spotted the glow of Kuala Lumpur and perhaps the Petronas Towers in the distance.

At 1:07 a.m., as the jet approached the east coast of Peninsular Malaysia, ground crews received what the authorities have described as a routine text message from the Aircraft Communications Addressing and Reporting System, or ACARS, which sends regular updates on the condition of the plane by radio or satellite.

By then, the first beverage service of the flight was most likely underway – soft drinks, juice and peanuts in economy; in business, various drinks, including Champagne, along with a snack such as charcoal-grilled chicken and lamb satay in peanut sauce. The in-flight entertainment system offered a wide selection of movies.

Air traffic control in Subang, outside Kuala Lumpur, followed the plane by tracking its transponder, a device that "squawks" or emits an identifying signal in response to a signal from radar. The night began with Flight 370 squawking code 2157 and reporting altitude, speed and bearing.

As the plane approached Vietnamese airspace, Subang informed the pilots that they were being transferred to radar control in Ho Chi Minh City. At 1:19 a.m., a voice identified by the authorities as that of the first officer, Fariq, replied, "All right, good night."

Two minutes later, Flight 370’s transponder stopped responding. It is unclear whether someone turned a dial on an instrument panel between the pilot and co-pilot and put the transponder in standby mode, or whether a malfunction caused it to go quiet.

One moment, radar scopes showed the plane traveling northwest at 471 knots (542 mph). The next moment, it was gone.

The military in Vietnam marked the time at 43 seconds past 1:20 a.m.

MYSTERY SIGNAL

As air traffic controllers struggled to re-establish contact with Flight 370, military radar at the Butterworth air force base on Malaysia’s west coast picked up an unidentified aircraft near where the plane disappeared.

But the watch team, normally an officer and three enlisted personnel, either failed to notice the signal or decided not to designate and track it as a "zombie," which would have pushed the information up the chain of command and possibly alerted air command.

At a briefing on the base the next night, about 80 air force personnel were told there was "no proof" the unidentified signal showed the missing plane making a sharp turn, flying back across Peninsular Malaysia and then turning again and heading northwest over the Strait of Malacca, a person familiar with the situation said.

But investigators now believe that is exactly what happened.

The failure to recognize Flight 370 in the radar data – or refusal to do so, to avoid the embarrassment of admitting an unidentified plane had breached air defense – meant the Malaysian authorities continued to search in the seas to the east instead of the west of the peninsula. Military radar last recorded the signal at 2:22 a.m., about 200 nautical miles northwest of Butterworth, according to an image of the radar track.

The authorities also failed to move quickly on data that showed the plane continuing to fly nearly seven more hours: a series of regular handshake signals from the plane to a satellite seeking to determine if the aircraft was still in range.

Chris McLaughlin, a vice president at Inmarsat, the satellite communications firm, said technicians pulled the logs of all transmissions from the plane within four hours of its disappearance. Then, after a day without sign of the plane, they began scouring the company’s databases for any trace of Flight 370.

"We decided to go have another look at our network to see if there was any data that we had missed," McLaughlin said. It turned out there was. Inmarsat technicians identified what appeared to be a series of fleeting "pings" between Flight 370, a satellite over the Indian Ocean and a ground station in Perth, Australia.

The signals – seven of them transmitted at one-hour intervals – were an important clue, because they could have come only from an antenna receiving power from the plane itself. But while they carried a unique code identifying the aircraft as Flight 370, the signals contained no positioning or other data that could indicate where the plane was when it sent them.

By Sunday afternoon, a team of Inmarsat engineers set to work using the principles of trigonometry to determine the distance between the satellite and the plane at the time of each ping, and then to calculate two rough flight paths. The plane, they concluded, had turned again. But it may have then traveled in more or less a straight line, heading north over countries likely to have picked it up on radar, or south toward the Indian Ocean and Antarctica.

The Malaysian government said it received Inmarsat’s data on March 12 and spent three days analyzing and vetting it with investigators from the United States before redirecting the search on March 15.

By then, more than a week had passed since the last satellite ping, recorded at 8:11 a.m. on March 8. It appears to have come from over the southern Indian Ocean, halfway around the world from where the plane should have been, on a tarmac in Beijing.

Philip P. Pan and Kirk Semple, New York Times

© 2014 The New York Times Company