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 cellphones and air phones 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,” said Ahmad Jauhari Yahya, the chief executive of Malaysia Airlines, at a news conference on 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.