The pilot of Malaysia Airlines flight MH370 was in control of the plane "until the end", French investigators reportedly suspect, after gaining access to "crucial" flight data.
The readouts "lend weight" to suspicions that he crashed into the sea in a murder-suicide, they were cited as saying.
The revelations based on Boeing data came days after a new account suggesting the pilot may have been clinically depressed, leading him to starve the passengers of oxygen and then crash the Boeing 777 into the sea.
MH370 was on its way from Kuala Lumpur to Beijing on March 8, 2014, with 239 people on board, when it vanished and became one of the world's greatest aviation mysteries.
In July last year investigators released a 495-page report, saying the plane's controls were probably deliberately manipulated to take it off course but they were not able to determine who was responsible.
The only country still conducting a judicial inquiry into the crash is France, where two investigating magistrates are looking into the deaths of three French passengers, the wife and two children of Ghyslain Wattrelos - an engineer who met the judges on Wednesday.
According to Le Parisien, they informed him that Boeing had finally granted them access late May to vital flight data at the plane maker's headquarters in Seattle.
This included numerous documents and satellite data from Britain-based company Immarsat.
They were obliged to sign a confidentiality contract, meaning the documents cannot be cited in court. The investigators also visited Immarsat headquarters in the UK.
It will take "a year" to sift through all the data and "nothing permits us to say the pilot was involved," according to the plaintiffs' lawyer, Marie Dosé.
However, French investigators cited by Le Parisien said the data "lends weight' to the idea that "someone was behind the control stick when the plane broke up in the Indian Ocean".
It cited a source close to the inquiry as saying someone was flying the plane "until the end."
"Certain abnormal turns made by the 777 can only have been carried out manually. Someone was in control," the source was cited as saying.
Asked whether the data pointed to a deliberate crash, the source said: "It's too early to assert it categorically but there is nothing to suggest anyone else entered the cockpit."
Wattrelos, who lost family members in the crash, hailed the "incredible" work of the judges, who he said "were able to note that the case was riddled with incoherences".
"For example, we know that the data initially provided by Malaysian authorities on the plane's altitude were wrong. And I hope that by analysing all the data collected at Boeing, they will discover a problem that will jump out at them," he told Le Parisien.
But he said he remained convinced that the plane was "taken down". "I don't know why or where but I'm convinced of it," he said. Wattrelos said that French investigators could meet FBI agents to discuss the case "over the summer in Paris".
More than 30 bits of suspected washed up debris have been collected from various places around the world.
Last month, friends of the pilot, Zaharie Ahmad Shah, 53, told aviation specialist William Langewiesche that he had become obsessed with two young models he had seen on the internet after his wife left him, and that he "spent a lot of time pacing empty rooms."
Langewiesche wrote: "There is a strong suspicion among investigators in the aviation and intelligence communities that he was clinically depressed."
An electrical engineer quoted in the account in The Atlantic magazine said that, after depressurising the plane, the pilot probably made a climb which "accelerated the effects of depressurising, causing the rapid incapacitation and death of everyone in the cabin."
The oxygen masks in the main cabin were only designed to last 15 minutes in an emergency descent below 13,000ft.
The pilot, however, would have had access to oxygen in the cockpit and could have flown for hours.
Writing in the Atlantic, Langewiesche said: "The cabin occupants would have become incapacitated within a couple of minutes, lost consciousness, and gently died without any choking or gasping for air."
One theory claims the pilot conducted a series of manouveres to "ditch the plane" - but some experts argue it would have been impossible for him to remain conscious during the emergency landing.
Pater Foley, the head of the Australian Transport Safety Bureau (ATSB), has suggested to the Australian Senate the pilot was unconscious when the plane crashed into the Indian ocean.
Foley said: "Today we have an analysis of the flap that tells us it is probably not deployed.
"We have an analysis of the final two transmissions that say the aeroplane was in a high rate of descent.
"We have 30 pieces of debris, some from inside the fuselage, that says there was significant energy at impact ... We have quite a lot of evidence to support no control at the end."
He added: "We haven't ever ruled out someone intervening at the end. It's unlikely."
A modern mystery: Malaysia Airlines Flight 370
Since Malaysia Airlines Flight MH370 vanished in March 2014, while flying from Kuala Lumpur International Airport, Malaysia, to its destination, Beijing Capital International Airport in China, conspiracy theorists have speculated that:
1. It was shot down: According to this theory, Flight 370 could have been shot down during a joint military exercise between the United States and Thailand in the South China Sea. The theory is advanced in a book called Flight MH370: The Mystery by Nigel Cawthorne, a UK-based writer. The book describes the plane's disappearance as "the greatest mystery since the Mary Celeste".
2. It was hijacked: With the 13th anniversary of the 9/11 attacks on New York and Washington approaching, conspiracy theorists have turned much of their focus to what some have dubbed the "9/11/14" theory. This theory, utterly devoid of proof but nonetheless widely circulated on the internet, suggests that Flight 370 did not crash but is in fact being prepared for use in a terrorist spectacular that will coincide with anniversary of the attacks on the World Trade Center and the Pentagon.
3. It was switched: The "plane switch" is among the most outlandish theories to have surfaced. One of its main backers is Jim Stone, a self-described "independent journalist" whose website carries the motto: "Truth is reality, which lies and inventions fall to in the end." Mr Stone proposes the theory that it was Malaysia Airlines Flight 370 and not Flight 17 that was shot down over the Ukraine in July last year. He offers little in the way of explanation about who might benefit from switching the planes, or how they might have achieved it.
4. It landed on an island: In the days and weeks following the plane's disappearance one of the most commonly heard conspiracy theories was that it had landed on Diego Garcia, the British-owned island in the Indian Ocean that is home to a major US military base. The theory gained so much traction that US officials were forced to deny it.
5. It was destroyed by a mysterious new weapon: Mike Adams, a veteran conspiracy theorist who runs a website called the Natural News, is known for his bizarre and often ludicrous articles. "There are some astonishing things you're not being told about Malaysia Airlines Flight 370," he wrote less than 48 hours after the disaster. His explanation for how MH370 "utterly and inexplicably vanished"? Not a hijacking nor a mid-air explosion but possibly the work of "some entirely new, mysterious and powerful force" capable of plucking "airplanes out of the sky without leaving behind even a shred of evidence".
6. Russia hijacked the plane: Under Putin's orders and flew it to Kazakhstan.
The reality check
Despite all those theories, the reality is certainly more mundane, says David Learmount, the aviation expert. He is convinced that whatever happened to the plane was "a deliberate act by someone on board, probably the captain". "I know people - especially journalists - would like this weird event to have more in common with Star Wars, but actually it was probably a carefully planned suicide and revenge attack," he said. "But will we ever be able to prove that? No."