SEPANG, Malaysia >> When hijackers took control of four airplanes on Sept. 11, 2001, and sent them hurtling low across the countryside toward New York and Washington, frantic passengers and flight attendants turned on their cellphones and began making calls to loved ones, airline managers and the authorities.

But when Malaysia Airlines Flight 370 did a wide U-turn in the middle of the night over the Gulf of Thailand and then spent nearly half an hour swooping over two large Malaysian cities and various towns and villages, there was apparently silence. As far as investigators have been able to determine, there have been no phone calls, Twitter or Weibo postings, Instagram photos or any other communication from anyone aboard the aircraft since it was diverted.

There has been no evidence "of any number they’re trying to contact, but anyway they are still checking and there are millions of records for them to process," Ahmad Jauhari Yahya, chief executive of Malaysia Airlines, said at a news conference Monday.

The apparent absence of any word from the aircraft in an era of nearly ubiquitous mobile communications has prompted considerable debate among pilots, telecommunications specialists and others. Most of the people aboard the plane were from Malaysia or China, two countries where mobile phone use is extremely prevalent, especially among affluent citizens who take international flights.

Some theorize the silence signifies that the plane was flying too high for personal electronic devices to be used. Others wonder whether people aboard the flight even tried to make calls or send messages.

According to military radar, the aircraft was flying extremely high shortly after its turn — as much as 45,000 feet, above the certified maximum altitude of 43,100 feet for the Boeing 777-200. It then descended as it crossed Peninsular Malaysia, flying as low as 23,000 feet before moving up to 29,500 feet and cruising there.

Vincent Lau, an electronics professor specializing in wireless communications at Hong Kong University of Science and Technology, said the altitude might have prevented passengers’ cellphones from connecting to base stations on the ground even if the phones were turned on during the flight or had been left on since departure.

The hijacked planes on Sept. 11 were flying very low toward urban targets when passengers and flight attendants made calls from those aircraft, he said.

Base station signals spread out considerably over distance. So cellphones in a plane a few miles up, like Flight 370, would receive little if any signal, he said.

Base station design has improved since the Sept. 11 attacks to provide better, more focused coverage of specific areas on the ground. But that also means somewhat less signal intensity is wasted in directions where callers are unlikely to be located, such as directly overhead, Lau added.

Lam Wong-hing, a wireless communications specialist at the University of Hong Kong , said cellphones transmit at 1 watt or less, while base stations typically transmit at 20 watts and sometimes much more. So even if a cellphone showed that it was receiving a signal while aloft, it might not be able to transmit a signal that was strong enough to make a connection, he said.

The metal in an aircraft reduces cellphone signals somewhat. If a passenger had pressed a cellphone against a plastic window with a line of sight to a cellphone tower it is possible a connection might have been made even at a fairly high altitude, because plastic barely blocks a cellphone signal at all, Lam said.

Many aircraft carry satellite phones, and the Malaysia Airlines jet was equipped with them in business class. The plane continued to send satellite pings for nearly seven hours after it was apparently diverted.

But the satellite phones are part of an aircraft’s in-flight entertainment system. If someone deliberately diverted a plane and turned off its transponder and other communications equipment, that person is likely to have disabled the in-flight entertainment system so passengers could not figure out from the map that they were flying in the wrong direction, said a telecommunications expert who insisted on anonymity because he was not authorized to talk to the media.

If the entertainment system was turned off, the satellite phones also would not work, the expert said.

Chinese media have reported that there have been some instances of people calling cellphones of passengers of the missing flight and hearing ring tones, sometimes days after the plane disappeared. Telecom experts have dismissed that as evidence that the cellphones are still in use, saying that a ring tone may be heard while the international phone system is searching for a phone and trying to connect a call.

There have been no reports of anyone answering calls to the cellphones of passengers or flight attendants aboard the plane.

Investigators do not know if anyone aboard the plane even tried to make a call. One theory is that someone may have intentionally depressurized the plane as it soared to an unusually high altitude right after the turnaround, which would have quickly rendered passengers and flight attendants unconscious, pilots said. Whoever diverted the plane could have disabled the release of oxygen masks.

Dr. James Ho, an associate professor of medicine at Hong Kong University, said death could come within minutes if someone were the equivalent of outdoors at 45,000 feet. But without information on the speed of depressurization, it is hard to predict the medical consequences, he said.

A table used by pilots for "time of useful consciousness" without an oxygen supplement at various altitudes shows only nine to 15 seconds at 45,000 feet, compared with five to 10 minutes at 22,000 feet.

Mobile phone service is widely available in sizable areas of western China and eastern Kazakhstan, raising the question of why nobody from the plane has tried to make a call if it did fly north and land safely, instead of flying out into the Indian Ocean until it ran out of fuel.

If the flight did land safely somewhere with the passengers and flight crew still healthy, whoever was in charge of the aircraft would also face a formidable task in any attempt to provide food, water and shelter for more than 200 people.