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U.S. investigators suspect that Malaysia Airlines 3786.KU -2.04% Flight 370 stayed in the air for about four hours past the time it reached its last confirmed location, according to two people familiar with the details, raising the possibility that the plane could have flown on for hundreds of additional miles under conditions that remain murky.

Aviation investigators and national security officials believe the plane flew for a total of five hours based on data automatically downloaded and sent to the ground from the Boeing Co. BA -0.99% 777's engines as part of a routine maintenance and monitoring program.

That raises a host of new questions and possibilities about what happened aboard the widebody jet carrying 239 people, which vanished from civilian air-traffic control radar over the weekend, about one hour into a flight to Beijing from Kuala Lumpur.

Six days after the mysterious disappearance prompted a massive international air and water search that so far hasn't produced any results, the investigation appears to be broadening in scope.

U.S. counterterrorism officials are pursuing the possibility that a pilot or someone else on board the plane may have diverted it toward an undisclosed location after intentionally turning off the jetliner's transponders to avoid radar detection, according to one person tracking the probe.

The investigation remains fluid, and it isn't clear whether investigators have evidence indicating possible terrorism or espionage. So far, U.S. national security officials have said that nothing specifically points toward terrorism, though they haven't ruled it out.

But the huge uncertainty about where the plane was headed, and why it apparently continued flying so long without working transponders, has raised theories among investigators that the aircraft may have been commandeered for a reason that appears unclear to U.S. authorities. Some of those theories have been laid out to national security officials and senior personnel from various U.S. agencies, according to one person familiar with the matter.

At one briefing, according to this person, officials were told investigators are actively pursuing the notion that the plane was diverted "with the intention of using it later for another purpose."

As of Wednesday it remained unclear whether the plane reached an alternate destination or if it ultimately crashed, potentially hundreds of miles from where an international search effort has been focused.

In those scenarios, neither mechanical problems, pilot mistakes nor some other type of catastrophic incident caused the 250-ton plane to mysteriously vanish from radar.

The latest revelations come as local media reported that Malaysian police visited the home of at least one of the two pilots.

Boeing officials and a Malaysia Airlines official declined to comment.

The engines' onboard monitoring system is provided by their manufacturer, Rolls-Royce RR.LN -1.71% PLC, and it periodically sends bursts of data about engine health, operations and aircraft movements to facilities on the ground.

Rolls-Royce couldn't immediately be reached for comment.

As part of its maintenance agreements, Malaysia Airlines transmits its engine data live to Rolls-Royce for analysis. The system compiles data from inside the 777's two Trent 800 engines and transmits snapshots of performance, as well as the altitude and speed of the jet.

Those snippets are compiled and transmitted in 30-minute increments, said one person familiar with the system. According to Rolls-Royce's website, the data is processed automatically "so that subtle changes in condition from one flight to another can be detected."

The engine data is being analyzed to help determine the flight path of the plane after the transponders stopped working. The jet was originally headed for China, and its last verified position was half way across the Gulf of Thailand.

A total flight time of five hours after departing Kuala Lumpur means the Boeing 777 could have continued for an additional distance of about 2,200 nautical miles, reaching points as far as the Indian Ocean, the border of Pakistan or even the Arabian Sea, based on the jet's cruising speed.

Earlier Wednesday, frustrations over the protracted search for the missing plane mounted as both China and Vietnam vented their anger over what they viewed as poor coordination of the effort.

Government conflicts and national arguments over crises are hardly unique to the Flight 370 situation, but some air-safety experts said they couldn't recall another recent instance of governments publicly feuding over search procedures during the early phase of an international investigation.

Authorities radically expanded the size of the search zone Wednesday, which already was proving a challenge to cover effectively, but the mission hadn't turned up much by the end of the fifth day.

WSJ has confirmed that the pilot had the ability to manually turn off the transponder on Flight MH370. A mid-air catastrophe could have destroyed it. Why is the transponder so significant? WSJ's Jason Bellini has #TheShortAnswer.

Also on Wednesday, a Chinese government website posted images from Chinese satellites showing what it said were three large objects floating in an 8-square-mile area off the southern tip of Vietnam. The objects were discovered on Sunday , according to the website, which didn't say whether the objects had been recovered or examined.

Ten countries were helping to scour the seas around Malaysia, including China, the U.S. and Vietnam. Taiwanese vessels are expected to be on the scene by Friday, with India and Japan having also agreed to join the search soon.

In all, 56 surface ships were taking part in the search, according to statements issued by the contributing governments, with Malaysia providing 27 of them. In addition, 30 fixed-wing aircraft were also searching, with at least 10 shipboard helicopters available, mostly in the waters between Malaysia and Vietnam.

China's government was especially aggrieved. More than 150 of the 239 people on board are Chinese, and family members in Beijing have at times loudly expressed their frustration over the absence of leads.

More than a dozen Chinese diplomats met with Malaysian authorities in Kuala Lumpur on Wednesday as tension grew over the search.

"At present there's a lot of different information out there. It's very chaotic and very hard to verify," foreign ministry spokesman Qin Gang said in a regular press briefing. "We've said as long as there is a shred of hope, you can't give up."

The day before, Beijing pointedly pressed Malaysia to accelerate its investigation, which has been hampered by false leads on suspected debris and conflicting reports on radar tracking.

Vietnam on Wednesday suspended its search flights after conflicting reports from Malaysia that authorities had tracked the plane to the Strait of Malacca before it disappeared.

Gen. Rodzali Daud, Malaysia's air force chief, denied saying he had told local media that military radar facilities had tracked the plane there, saying they were still examining all possibilities. Vietnam later resumed normal search sweeps.

Malaysian authorities divided the search area into several sectors on either side of the country, as well as areas on land.

The challenge, said Lt. David Levy, a spokesman for the U.S. Navy's Seventh Fleet, isn't so much coordination as the sheer size of the area involved. The search grids are up to 20 miles by 120 miles, and ships and aircraft employ an exhaustive methodical pattern "like mowing your lawn" in their search for the plane, he said.

China and Vietnam venting their frustration with the slow progress of the mission and what they view as poor coordination of the effort to find Malaysia Airlines Flight 370. Allison Morrow reports on the News Hub. Photo: Getty Images.

U.S. defense officials sought to play down any suggestion that the Malaysian government was doing a poor job with the search.

"It is not unusual for searches to take a long time, especially when you are working with limited data," one official said.

Aviation experts say the absence of an electronic signal from the plane before it disappeared from radar screens makes it difficult to pin down possible locations. Some radar data suggested the Boeing 777 might have tried to turn back to Kuala Lumpur before contact was lost, a detail that prompted a search for the plane on both sides of the Malaysian peninsula.

A U.S. Navy P-3C Orion maritime patrol aircraft has been searching the northern Strait of Malacca, west of Malaysia, while destroyers USS Kidd and USS Pinckney have been deploying helicopters in the Gulf of Thailand to the east.

So far the U.S., like other nations taking part in the search, has had no success. Many aviation experts are concluding that searchers might not have been looking in the right places. Even if the plane broke up in midair, it would have left telltale traces of debris in the ocean. The cracks now emerging between some of the participants in the search could make it even more difficult.

You can help search for the missing Malaysia Airlines plane, thanks to a website called Tomnod.com. It allows anyone to comb the area where rescue workers are searching using satellite images. The WSJ's Deborah Kan speaks to DigitalGlobe's Luke Barrington.

Diplomatic feuds over air disasters have generally erupted over the conclusions of the investigations, long after the initial search is over.

The results of the 1999 crash of an Egyptair Boeing 767 en route to Egypt from New York, which killed 217 people, spawned a dispute between Washington and Cairo that strained ties for years. The National Transportation Safety Board concluded the plane's co-pilot purposely put the twin-engine jet into a steep dive and then resisted efforts by the captain to recover control before the airliner slammed into the Atlantic Ocean off the coast of Nantucket. Egyptian authorities insisted the evidence indicated mechanical failure.

Years earlier, Washington and Paris butted heads over the investigation of an American Eagle commuter turboprop that crashed in 1994 near Roselawn, Ind. The French objected to the NTSB's conclusions that French regulators failed to take actions that could have prevented the accident.