Story highlights "This is beginning to look very, very much like a hijacking," says John Nance

"Mechanical explanations are narrowing quickly," says former safety board managing director

Transponders: "They're not supposed to break, but they do break"

As investigators search for clues to what happened to Malaysia Airlines Flight 370, the answer to one question may prove key: Why did the transponder in the Boeing 777-200ER stop transmitting information?

The fact that it happened at all is astonishing to John Nance, a broadcast aviation analyst and veteran pilot. "It is hard to conceive of a situation in which a triple seven would lose all ability to have its transponder on and the crew would not find some way to communicate," he told CNN.

A senior Malaysian air force official said Tuesday that the plane traveled hundreds of miles in the opposite direction from its original destination, and had stopped sending identifying transponder codes before it disappeared from radar screens.

Suggestions that the plane veered off course and that its transponder was not working raise questions about a hijacking, but a catastrophic power failure or other problem might also explain the anomalies, analysts said.

Here are some of the options:

Photos: The search for MH370 Photos: The search for MH370 Two years after Malaysia Airlines Flight 370 went missing, a relative of one of the passengers burns incense in Beijing on March 8, 2016. Flight 370 vanished on March 8, 2014, as it flew from Kuala Lumpur, Malaysia, to Beijing. There were 239 people on board. Hide Caption 1 of 43 Photos: The search for MH370 On July 29, police carry a piece of debris on Reunion Island, a French territory in the Indian Ocean. A week later, authorities confirmed that the debris was from the missing flight. Hide Caption 2 of 43 Photos: The search for MH370 Staff members with the Australian Transport Safety Bureau examine a piece of aircraft debris at their laboratory in Canberra, Australia, on July 20. The flap was found in June by residents on Pemba Island off the coast of Tanzania, and officials had said it was highly likely to have come from Flight 370. Experts at the Australian Transport Safety Bureau, which is heading up the search for the plane, confirmed that the part was indeed from the missing aircraft. Hide Caption 3 of 43 Photos: The search for MH370 In late February, American tourist Blaine Gibson found a piece of plane debris off Mozambique, a discovery that renewed hope of solving the mystery of the missing flight. The piece measured 35 inches by 22 inches. A U.S. official said it was likely the wreckage came from a Boeing 777, which MH370 was. Hide Caption 4 of 43 Photos: The search for MH370 Relatives of the flight's passengers console each other outside the Malaysia Airlines office in Subang, Malaysia, on February 12, 2015. Protesters had demanded that the airline withdraw the statement that all 239 people aboard the plane were dead. Hide Caption 5 of 43 Photos: The search for MH370 A police officer watches a couple cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The couple's son was on the plane. Hide Caption 6 of 43 Photos: The search for MH370 Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27, 2014. Data from communications between satellites and the missing flight was released the day before, more than two months after relatives of passengers said they requested it be made public. Hide Caption 7 of 43 Photos: The search for MH370 Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14, 2014. Hide Caption 8 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13, 2014. Hide Caption 9 of 43 Photos: The search for MH370 The HMS Echo, a vessel with the British Roya; Navy, moves through the waters of the southern Indian Ocean on April 12, 2014. Hide Caption 10 of 43 Photos: The search for MH370 A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9, 2014. Hide Caption 11 of 43 Photos: The search for MH370 A relative of a missing passenger cries at a vigil in Beijing on April 8, 2014. Hide Caption 12 of 43 Photos: The search for MH370 Australian Defense Force divers scan the water for debris in the southern Indian Ocean on April 7, 2014. Hide Caption 13 of 43 Photos: The search for MH370 A towed pinger locator is readied to be deployed off the deck of the Australian vessel Ocean Shield on April 7, 2014. Hide Caption 14 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4, 2014. Hide Caption 15 of 43 Photos: The search for MH370 On March 30, 2014, a woman in Kuala Lumpur prepares for an event in honor of those aboard Flight 370. Hide Caption 16 of 43 Photos: The search for MH370 The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, 2014, after other relatives left en masse to protest the Malaysian government's response to their questions. Hide Caption 17 of 43 Photos: The search for MH370 A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27, 2014. Hide Caption 18 of 43 Photos: The search for MH370 Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27, 2014. Hide Caption 19 of 43 Photos: The search for MH370 People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27, 2014. Hide Caption 20 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24, 2014. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived." Hide Caption 21 of 43 Photos: The search for MH370 Grieving relatives of missing passengers leave a hotel in Beijing on March 24, 2014. Hide Caption 22 of 43 Photos: The search for MH370 A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22, 2014. Hide Caption 23 of 43 Photos: The search for MH370 A Chinese satellite captured this image, released on March 22, 2014, of a floating object in the Indian Ocean, according to China's State Administration of Science. It was a possible lead in the search for the missing plane. Surveillance planes were looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia. Hide Caption 24 of 43 Photos: The search for MH370 Satellite imagery provided by the Australian Maritime Safety Authority on March 20, 2014, showed debris in the southern Indian Ocean that could have been from Flight 370. The announcement by Australian officials raised hopes of a breakthrough in the frustrating search. Hide Caption 25 of 43 Photos: The search for MH370 Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight. Hide Caption 26 of 43 Photos: The search for MH370 A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19, 2014. Hide Caption 27 of 43 Photos: The search for MH370 On March 18, 2014, a relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet. Hide Caption 28 of 43 Photos: The search for MH370 U.S. Navy crew members assist in search-and-rescue operations in the Indian Ocean on March 16, 2014. Hide Caption 29 of 43 Photos: The search for MH370 Members of the Chinese navy continue search operations on March 13, 2014. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, search efforts expanded west into the Indian Ocean. Hide Caption 30 of 43 Photos: The search for MH370 A Vietnamese military official looks out an aircraft window during search operations March 13, 2014. Hide Caption 31 of 43 Photos: The search for MH370 Malaysian air force members look for debris near Kuala Lumpur on March 13, 2014. Hide Caption 32 of 43 Photos: The search for MH370 Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12, 2014. Hide Caption 33 of 43 Photos: The search for MH370 A member of the Vietnamese air force checks a map while searching for the missing plane on March 11, 2014. Hide Caption 34 of 43 Photos: The search for MH370 A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported on March 8, 2014. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10, 2014. Hide Caption 35 of 43 Photos: The search for MH370 A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews on March 9, 2014, before returning to search for the missing plane in the Gulf of Thailand. Hide Caption 36 of 43 Photos: The search for MH370 Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9, 2014. Hide Caption 37 of 43 Photos: The search for MH370 Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9, 2014. Hide Caption 38 of 43 Photos: The search for MH370 The rescue vessel sets out from Sanya in the South China Sea on March 9, 2014. Hide Caption 39 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8, 2014. Hide Caption 40 of 43 Photos: The search for MH370 A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8, 2014. Hide Caption 41 of 43 Photos: The search for MH370 Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8, 2014. Hide Caption 42 of 43 Photos: The search for MH370 Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference at a hotel in Sepang on March 8, 2014. "We deeply regret that we have lost all contacts" with the jet, he said. Hide Caption 43 of 43

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Turned off intentionally

Kit Darby, a longtime pilot, said Tuesday it was not clear whether the transponder was turned off intentionally. A power failure would have turned off the main transponder and its backup, and the plane could have flown for more than an hour with such a power failure, the president of Aviation Information Resources told CNN's Wolf Blitzer.

But Nance expressed doubt that that could have been the case. The electrical system aboard the plane is so robust and the transponder draws so little power that it would be one of the last pieces of equipment to go dark, even after a catastrophic event like an engine explosion or a breach of the cabin and rapid decompression, he said.

"I'm in a head-scratching mode," Nance said. "The most likely probability is that a human hand turned that off. Then you get into the logic tree of who and why and there aren't that many channels in that tree."

He added, "This is beginning to look very, very much like a hijacking."

A former Federal Aviation Administration safety inspector agreed. David Soucie, author of "Why Planes Crash," cited the redundant electrical, charging, battery and communications systems on Boeing 777s. Much had to go wrong for the aircraft to lose its transponder and then to veer off course, he said, adding that it stands to reason "that someone forced those pilots to take control of the aircraft and take it off course."

Turning off a transponder requires a deliberative process, said Peter Goelz, former managing director of the National Transportation Safety Board. "If someone did that in the cockpit, they were doing it to disguise the route of the plane," he told CNN. "There might still be mechanical explanations on what was going on, but those mechanical explanations are narrowing quickly."

'Patchy' radar

Alastair Rosenschein suggested that a pressurization problem may have been to blame. If the plane lost pressure and the pilots failed to don their masks within a few seconds, "they would become unconscious," the aviation consultant and former British Airways Boeing 747 pilot told CNN. "The aircraft would continue on the last heading."

If that happened, the plane may have crashed in the middle of the Indian Ocean, nowhere near where the search is going on, he noted.

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How does he explain the loss of the transponder signal?

"It is possible that, even with the transponder putting out a signal, the radar controllers didn't notice it, pay attention to it or receive it," he speculated. That would be rare, he acknowledged, but so too is the disappearance of a jetliner.

And radar is "pretty patchy" around there, he said.

The transponder broke

The mechanics of the device may have been at fault, said Kirk Fryar, president of Sarasota Avionics, which sells the devices. "They're not supposed to break, but they do break," he told CNN. "Sometimes the transponder itself could be off frequency, not sending the right pulse."

The boxes are located in the cockpit, within reach of the pilot, copilot or both, he said. Each is equipped with an on, standby or off mode.

And there is often more than one. "Boeing would have at least two transponders," he said. "What happens is sometimes you're flying along and, say, your transponder breaks and reports the wrong code or wrong altitude, air traffic control will go, 'You need to turn it off because we're getting erroneous readings,'" he said.

And, if one were to break, the pilot or copilot would have to flip a switch to replace it, something that a pilot stressed during an emergency might not do.

An antenna malfunction may also have been to blame, but that would be unlikely on a Boeing, which undergoes routine maintenance checks, Fryar said.

Flying below the radar

Flying below the line of sight of the air traffic controller is another way for a transponder to stop identifying a plane. "The higher you are, the better sensitivity you're going to have to air traffic control," he said.

That would be improbable during a routine flight from Kuala Lumpur to Beijing, the itinerary that the Boeing jet was on when it fell off radar early Saturday.

He too cited the possibility that it was turned off intentionally. Why might that have happened? To escape detection. Fryar recounted when he and his flying instructor flew into airspace over Denver without having communicated with air traffic control.

"He goes, 'Oh, my God, we're going to get in trouble,' flew 500 feet off the ground, turned off the transponder. They can't find you, basically." Thirty miles later, he switched it back on and the two completed their flight without incident.

CNN Law Enforcement Analyst Tom Fuentes said Tuesday he was not persuaded by any of the theories: "There's still as many possibilities out there, maybe more, now that we know about the transponders being off and the length of time that plane flew in the air without them. It still leaves mechanical, terrorism (and) other issues as much in the air as they were before."

History

Transponders have been around since World War II and commonly used in general aviation since the late 1960s, said Fryar.

It supplemented military radar, which was unreliable -- unable to distinguish a flock of birds from a plane, he said.

A transponder for use in general aviation would typically cost from $2,000 to $7,000; versus $20,000-$30,000 for a commercial airliner.

There's nothing fancy about the technology. An air traffic controller sends out a radar sweep that contains a microwave pulse requesting information; the transponder decodes the request and sends back that information -- identity of the aircraft, its location, altitude and speed.

Primer:

Q. What information does a transponder send, and who does it send it to?

A transponder is a radio transmitter in the cockpit that works with ground radar. When the transponder receives a signal from a more sophisticated ground "secondary" radar, it returns a squawk code with the aircraft's position, its altitude and its call sign. It is constantly being radar pinged, helping air traffic controllers on the ground determine the airplane's speed and direction, too.

Q. What does "squawk" mean?

It's a four-digit identifying code that the pilot enters into a transponder for each flight. It helps air traffic control recognize each plane.

Q. Why are transponders necessary?

Primary radar is more basic, effective only at seeing the radar reflection of objects. It paints targets, displaying them with a blip on a screen. The bigger the object, the bigger the reflection. Transponders enhance the reflected signal and provide the additional information for air traffic control.

Q. Is there a way to use it to indicate a problem on board the aircraft?

There are codes for different emergencies. For a hijacking, it's 7500. For communications failure, 7600. Emergencies are 7700.

Q. How do you turn off a transponder?

There is a switch that you would move from "ON" or "SBY" (standby) or "ALT" (altitude). You could also pull the circuit breaker for the transponder in the cockpit.

Q. Why would you turn off a transponder during a normal flight?

There could be several reasons. One reason could be when airplanes get close to each other (perhaps they are approaching an airport). Air traffic controllers may then request pilots to turn the transponders off or to standby. Also, if the transponder is sending faulty information, the pilot might want to turn it off. Planes are still visible on primary radar until they get below the radar's coverage ability.