KUALA LUMPUR, Malaysia »The retired chief pilot of Malaysia Airlines is torn between logic and loyalty to an old friend. Nik Huzlan, 56, was one of the first captains to fly the 12-year-old Boeing 777 that disappeared over the Indian Ocean a year ago this Sunday. He knew the pilot who flew the plane that day, Zaharie Ahmad Shah, for decades.

Huzlan is convinced that deliberate human intervention, most likely by someone in the cockpit, caused the aircraft, on a red-eye flight from Kuala Lumpur to Beijing, to suddenly turn around, cease communication with air traffic control and some six hours later run out of fuel and fall into the ocean. But he also said he had never seen anything in more than 30 years of friendship that would suggest that Zaharie was capable of such a deed.

"Based on logic, when you throw emotion away, it seems to point a certain direction which you can’t ignore," Huzlan said. "Your best friend can harbor the darkest secrets."

No trace of the plane has been found, although four ships continue to scour a section of the ocean floor roughly the size of West Virginia and as deep as 3 miles below the surface. Without the plane’s flight recorders, the disappearance remains a mystery.

But the "rogue pilot theory," as investigators call it, has emerged as the most plausible explanation among several. Many, but not all, of the investigators and experts who have reviewed the limited evidence say Zaharie, or perhaps the co-pilot, Fariq Abdul Hamid, is the likeliest culprit, although they caution that the evidence is limited and circumstantial and that the theory is full of holes, like lack of a motive.

"I would say that’s my favorite, because it would fit best with what has happened," said Peter Marosszeky, a longtime Australian airline executive who is a senior research fellow at the University of New South Wales. But he added that without finding and retrieving at least part of the plane, it would be hard to say anything conclusively.

Others still offer a different explanation for the disappearance of a jumbo jet with 239 people aboard: mechanical failure, a fire, hijacking, sabotage or some other event as yet unknown.

Psychological profiles of the pilot prepared after the disappearance of Flight 370 do not suggest Zaharie could have taken the plane down or would have had a compelling reason for doing so, several people with detailed knowledge of the investigation said. His family has emphatically denied that he would have deliberately turned the plane around and flown it to its destruction.

A rival theory in the early days after the plane’s disappearance, a midair equipment failure, falls apart for lack of a breakdown that could swiftly disable separate communications systems but still allow the plane to stay in the air and perform a long series of maneuvers.

There were no reports of bad weather in the area.

Yet at 1:21 a.m. that March 8, 40 minutes into the flight, all communication with the aircraft was lost, and its radar label vanished from the screens of ground controllers. According to military radar, which continued to track the plane, it suddenly altered its northeasterly course, veering west and south, over the Malay Peninsula and across the island of Penang, where Zaharie grew up. It then headed out to sea across the Strait of Malacca before turning south into the Indian Ocean.

Why is a question that may not be answered until the wreckage is found, and possibly not even then. The Malaysian government is expected to release an accident report in the next several days that may provide more information.

That puts the focus on finding the aircraft. Search planes and ships have been scouring the ocean west of Australia since late March. Based on modeling from the aircraft’s electronic handshakes with a satellite positioned over the Indian Ocean, an Australian-led team narrowed the search area to a 23,000-square-mile swath of ocean, about 1,100 miles west-northwest of Perth, Australia.

Four ships under contract by the Australian and Malaysian governments are searching the site, braving swells reaching 55 feet as cyclone after cyclone churns the ocean between Africa and Australia. Crews work 12-hour days, with no days off, six weeks at a time.

The vessels are towing side-scan sonar devices that glide above the ocean floor at the end of armored fiber-optic cables up to 10,000 yards long, creating detailed maps of the ocean floor. They follow a pattern like mowing a lawn, heading back and forth in the search box to cover every square yard.

They have scoured nearly half the area, and they expect to complete the job by May.

"We still have pretty good confidence that we’ll find the aircraft in the priority search area," Martin Dolan, chief commissioner of the Australian Transport Safety Bureau, the agency leading the search, said in a telephone interview.

The Australian government has not begun consultations with other governments on what to do if they do not find the missing plane.

"I can’t promise that the search will go on at this intensity forever, but we will continue our very best efforts to resolve this mystery and provide some answers," Prime Minister Tony Abbott of Australia told Parliament in Canberra on Thursday.

Extensive analyses of the satellite pings sent by the plane in its final hours show that it was headed more or less due south until it ran out of fuel. Statistical models show that, in theory, it could have ended up anywhere in a 425,000-square-mile area, about the size of Texas and California combined. Dolan said it was "unlikely" the plane was outside the designated search area, however, because analyses of the satellite data suggest the plane was on autopilot as it flew south and made a steep descent at the end, consistent with a plane running out of fuel.

Paul Kennedy, the search director for Fugro, the contractor operating three of the ships, is confident that his vessels can find the aircraft. The company’s sonar equipment can detect objects as small as a meter wide, and a 777 extends more than 63 meters, or about 200 feet.

"The technical publications give you confidence that we are looking in the right place," Kennedy said. "It is just not possible for it to be anywhere else. Too many experts, around the world, independently analyzed the data and came to the same conclusion. These are seriously clever people."

For the relatives of the passengers, a year has been a long time to go without answers. Many complain that the Malaysian and Chinese governments — 153 of the 227 passengers were Chinese — have ignored them.

"No one is listening," said Steve Wang, whose mother was on the plane and who is an unofficial spokesman for the families in Beijing. "I cannot describe our rage."

Like much of the public, they cannot fathom how, in an era where a missing mobile phone can be located in moments, a wide-body jetliner could simply vanish.

That sense of vulnerability gave new impetus to a long-running debate within the aviation industry over how flights could be tracked more closely in order to help rescuers and investigators respond more quickly in an emergency.

"Flight 370 showed that in today’s very connected world, the idea that we cannot know where every airplane is at any given moment has become unacceptable," said Rimi Jouty, the director of the French Bureau of Investigations and Analysis, which has been advising investigators on the case.

French investigators had recommended closer flight tracking measures, including real-time streaming of flight data, in 2012, after a two-year search for the wreckage of an Air France jetliner that crashed in the Atlantic Ocean in 2009. But safety regulators were slow to respond with concrete proposals, viewing the likelihood of a similar event as extremely remote.

But it did happen again. And in the wake of the Flight 370 disappearance, the airlines and the International Civil Aviation Organization, a U.N. body, have agreed in principle on the need for all airliners to have the ability, by November 2016, to automatically report their position at least every 15 minutes, twice as often as the current average of around 30 minutes.

Had those measures been in place before Flight 370 took off from Kuala Lumpur Airport, they may not have prevented the plane from crashing, but the plane would have most likely been found by now and the question of what happened put to rest.

For his part, Huzlan is reluctant to definitively blame his old friend.

"Despite the trail of logic," he said he could not assume that his old friend "would, on his own accord, for whatever reason, lead 238 others whose lives he was entrusted to hold in his hand to their doom in the depths of the world’s loneliest place, the South Indian Ocean."

© 2015 The New York Times Company