Later in the day, Strobe Talbott, who was deputy secretary of state in the Bill Clinton administration and remains an informed and influential voice in the US capital, tweeted: “Malaysia plane mystery: Direction, fuel load & range now lead some to suspect hijackers planned a 9/11-type attack on an Indian city.”

At a news conference, Malaysia’s PM Najib Razak said his government would seek the help of other governments across a large region of Asia in trying to find the plane. Malaysian authorities later released a map showing that the last satellite signal received from the plane had been sent from a point somewhere along one of two arcs spanning large distances across Asia.

This map shows two red lines representing the possible locations from which Flight 370 sent its last hourly transmission to a satellite at 8.11am on March 8 — more than seven hours after it took off from KL and when the plane would most likely have been running low on fuel.

Najib said a satellite orbiting 35,800km over the middle of the Indian Ocean received a transmission that, based on the angle of transmission from the plane, came from a location somewhere along one of two arcs. One arc runs from the southern border of Kazakhstan in Central Asia to northern Thailand. The other runs from near Jakarta, Indonesia, to the Indian Ocean.

“These movements are consistent with deliberate action by someone on the plane,” Najib said. He noted that one communications system had been disabled as the plane flew over the northeast coast of Malaysia. A second system, a transponder aboard the aircraft, abruptly stopped broadcasting its location, altitude, speed and other information a few minutes later, at 1.21am, while the plane was one-third of the way across the Gulf of Thailand from Malaysia to Vietnam.

The information came a day after American officials and others familiar with the investigation told The New York Times that Flight 370 had experienced significant changes in altitude after it lost contact with ground control, and altered its course more than once as if still under the command of a pilot.

Military radar data subsequently showed that the aircraft turned and flew west across northern Malaysia before arching out over the wide northern end of the Strait of Malacca, headed at cruising altitude for the Indian Ocean. The disappearance of the jet has worried China, partly because nearly two-thirds of the 239 people aboard were Chinese citizens. On Saturday, the Chinese ministry of foreign affairs demanded to know more, and said that China was sending technical experts to Malaysia.

The flight had been scheduled to land at 6.30am in Beijing, so the time of the last satellite signal as given by Najib — 8.11am— could have been as the plane was nearing the end of its fuel supply.” The investigation team is making further calculations, which will indicate how far the aircraft may have flown after the last point of contact,” Najib said. “Due to the type of satellite data, we are unable to confirm the precise location of the plane when it last made contact with a satellite.”

The northern arc described by Najib passes through or close to some of the world’s most volatile countries, home to insurgent groups, but also over highly militarized areas with robust air-defence networks, some run by the American military. The arc passes close to northern Iran, through Afghanistan and northern Pakistan, and through northern India and the Himalayan mountains and Myanmar. An aircraft flying on that arc would have to pass through air-defence networks in India and Pakistan, whose mutual border is heavily militarized, as well as through Afghanistan, where the US and other Nato countries have operated air bases for more than a decade. These include Bagram Airfield in Afghanistan and a large Indian air base, Hindon Air Force Station.

The Indian Ocean, the third-largest in the world, has an average depth of more than 12,000 feet.

Mikael Robertsson, a founder of Flightradar24, a global aviation tracking service, said the way the plane’s communications were shut down pointed to the involvement of someone with considerable aviation expertise and knowledge of the air route, possibly a crew member, willing or unwilling.

The Boeing’s transponder was switched off just as the plane passed from Malaysian to Vietnamese air traffic control space, thus making it more likely that the plane’s absence from communications would not arouse attention, Robertsson said by telephone from Sweden.

“Always when you fly, you are in contact with air traffic control in some country,” he said. “Instead of contacting the Vietnam air traffic control, the transponder signal was turned off, so I think the timing of turning off the signal just after you have left Malaysian air traffic control indicates someone did this on purpose, and he found the perfect moment when he wasn’t in control by Malaysia or Vietnam. He was, like, in no-man’s country.”

Xu Ke, a former commercial pilot and now lecturer at the Zhejiang Academy of Police in eastern China who studies aviation security, said the details suggested that at least one member of the crew, most likely one of the pilots, was involved in seizing control of the aircraft, either willingly or under coercion. “The timing of turning off the transponder suggests that this involved someone with knowledge of how to avoid air traffic control without attracting attention,” Xu said in a telephone interview. “You needed to know this plane, and you also needed to know this route.”

Especially since the September 11 attacks, Xu said, security on cockpit doors has been reinforced so that it would be difficult for anyone to force their way in without giving the pilots ample time to send a warning signal. “We have to be careful about our words and conclusions, and examine all the possibilities, but the likelihood that a pilot was involved appears very likely,” said Xu. “The Boeing 777 is a relatively new and big plane, so it wouldn’t be anyone who could do this, not even someone who has flown smaller passenger planes, even smaller Boeings.”

According to a person who has been briefed on the progress of the investigation, the two “corridors” described by Mr. Najib were derived from calculations made by engineers from the satellite communications company Inmarsat, which were provided to investigators. The person spoke on the condition of anonymity because details of the search operation remain confidential.

But based on what is already known about the flight’s trajectory, investigators are strongly favouring the southern corridor as the likely flight path, the person said. “The US Navy would not be heading toward Kazakhstan,” the person said.