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 Inmarsat.
They were obliged to sign a confidentiality contract, meaning the documents cannot be cited in court. The investigators also visited Inmarsat 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.”
Mr 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. Mr 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.”
Mr 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, Mr 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.
Mr 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.”