Malaysia's leader says communications systems on Flight 370 were cut off by "deliberate action." U.S. officials are investigating whether a third system, on the plane's lower deck, was also compromised. WSJ's Jason Bellini explains. Image: AP

The investigation into the disappearance of Malaysia Airlines 3786.KU +2.13% Flight 370 sharpened its focus on sabotage, according to aviation and industry officials, amid strong indications that one or more people on the plane deliberately changed its course and tried to mask its location.

Officials suspect two different systems were shut off after the plane took off last weekend, one shortly after the other, people familiar with the investigation said. About an hour into the flight, the plane's transponders stopped functioning, making it much more difficult for air-traffic control personnel to track or identify it via radar.

Malaysian Prime Minister Najib Razak says investigators are looking at profiles of passengers and crew on Flight 370. The WSJ's Jeffrey Ng tells us where the investigation is headed next.

In the ensuing minutes, a second system sent a routine aircraft-monitoring message to a satellite indicating that someone made a manual change in the plane's direction, veering sharply to the west. Such a turn wouldn't have been part of the original authorized route programmed in the flight-management computer that controls the autopilot. Those system-monitoring messages are suspected to have been disabled shortly afterward, according to some of these people.

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"Increasingly, it seems to be heading into the criminal arena," said Richard Healing, a former member of the U.S. National Transportation Safety Board. The latest revelations about the investigation, he added, "indicate the emphasis is on determining if a hijacker or crew member diverted the plane."

Despite the efforts to hide the location of the Boeing 777 with 239 people on board,the plane kept broadcasting its location hourly via a satellite communication system for five more hours, according to several people familiar with the investigation. The last of these transmissions was sent from high above the Indian Ocean, according to two of these people.

Depending on the direction, speed and altitude of its flight, investigators estimate the jet could have had less than an hour's worth of fuel left at the point of the last satellite signal. But if the plane stayed high to be more efficient, it could have had significantly more fuel left at that point.

In assembling the chain of events, investigators are trying to determine why the jet didn't travel farther from its takeoff point before losing its satellite contact given the number of hours it apparently was in the air.

The international search has drastically expanded its mission westward, with the U.S. Navy and other nations now searching for the plane in a 320,000-square-mile rectangle west of the Andaman Islands.

The Malaysia Airlines Boeing 777 is seen in Paris in December. Associated Press

An official criminal investigation hasn't been opened, and an international team of investigators hasn't ruled out the possibility that some type of catastrophic event, pilot error or mechanical malfunction was the cause of the plane's disappearance. The Federal Bureau of Investigation has looked into the backgrounds of the passengers and pilots, a U.S. official said, but hasn't found any ties to terrorist groups or other indications they may have tried to hijack or sabotage a plane.

Still, as details emerge an accident appears increasingly unlikely. The first loss of the jet's transponder, which communicates the jet's position, speed and call sign to air traffic control radar, would require disabling a circuit breaker above and behind an overhead panel. Pilots rarely, if ever, need to access the circuit breakers, which are reserved for maintenance personnel.

A physical disconnection of the satellite communications system would require extremely detailed knowledge of the aircraft, its internal structure and its systems.

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WSJ has confirmed that the pilot had the ability to manually turn off the transponder on Flight MH370. Why is the transponder so significant? WSJ's Jason Bellini has #TheShortAnswer.

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Military officer Dang Xuan Hung looks out a window of a Vietnam Air Force aircraft during a mission to find the missing Malaysia Airlines flight MH370 that disappeared from radar screens in the early hours Saturday. Reuters

"Everything so far makes it seem as though someone was controlling the airplane" and attempting to fly it somewhere other than its intended destination, said Robert Francis, another former NTSB member. The longer the search goes on, he said, the less it seems to be "what you would expect from a civil-aviation aircraft in trouble."

Also emerging as a possible focus is whether more than one person on board the plane may have been involved in its disappearance.

The satellite pings stopped roughly five hours after the other systems stopped working, cutting off all identifying signals from the plane. Aviation investigators are trying to determine, among other things, whether someone would have had to climb into an electronics bay located on the plane's lower deck to disable that equipment.

It is also possible that the satellite communication gear, rather than being disabled, stopped sending pings because the plane had crashed some time after the final transmission.

U.S. aviation experts say one challenge, especially if a criminal probe were ordered, would be getting Malaysian government officials and investigators to avoid mixed messages, confusion and friction with other countries. A series of false leads and conflicting radar reports has prompted criticism of Malaysian investigators after a week of mostly fruitless searching.

Hong Lei, a Chinese foreign ministry spokesperson, chided Malaysia for not sharing information just three days after Beijing asked the country to accelerate its probe and speed up its search efforts on behalf of the families of passengers on the flight. Of the 239 passengers on the plane, more than 150 were Chinese.

Malaysian authorities said they are working with U.S. experts and will receive help from a British team, composed of the country's Air Accidents Investigation Branch and engine maker Rolls-Royce, said Azharuddin Abdul Rahman, director-general of Malaysia's Department of Civil Aviation. Defense and Acting Transport Minister Hishammuddin Hussein said investigators will probe why the plane's transponders, which send signals about the aircraft to identify it to radar, went off.

The NTSB has sent a trio of veteran accident investigators to Malaysia to help with the investigation. The group includes Scott Dunham, an expert in air traffic control and radars. The small size of the team partly reflects the fact that no wreckage has been recovered.

If a criminal probe is launched, the FBI is likely to send in a phalanx of agents and other personnel. Such a probe is likely to involve many countries, but aviation experts believe it would be headed by Malaysia because of the aircraft's nationality.

"The FBI would have full entrée into the case because of the American citizens on board," said Robert MacIntosh, who previously headed up international relations for the NTSB.

One U.S. official said Friday that pursuing an international criminal investigation would be "a huge challenge" because so many governments will have an interest in the process and outcome. So far, U.S. officials haven't found derogatory information on the pilots or anyone else on the flight manifest, the official said.

Some U.S. aviation experts question how smoothly any complex criminal investigation would proceed. Already, Mr. MacIntosh said, "certainly there has been competition between [Malaysian] civilian and military officials."

As the demand for answers heats up, Mr. Healing asked: "Are they prepared to deal with this kind of massive international investigation?"

—Julian E. Barnes, Andrew Grossman and Siobhan Gorman 
contributed to this article.