French investigators have uncovered a mysterious "third entity" which may be withholding technical data about the path taken by missing Malaysia Airlines Flight 370.
They have also identified a number of "curious passengers" aboard the Boeing 777 who they believe warrant further investigation.
The potentially explosive new developments were revealed by Ghyslain Wattrelos, a French national who lost his wife and two teenage children on MH370, following his meeting last week with judges overseeing the Gendamarie Air Transport (GTA) investigation.
Wattrelos said he was told the French team had found "inconsistencies" in the Malaysian investigation's official report and identified the presence of "curious" passengers, whom "we should continue to investigate".
They include a Malaysian national and aeronautics specialist seated directly under MH370's Satcom module who potentially had the technical knowledge to hack the plane's communication systems and disguise its route.
The GTA, an arm of the French military, is seeking to verify satellite and other technical data used by the Australian Transport Safety Bureau (ATSB) to plot the plane's journey to a remote part of the southern Indian Ocean off WA, where it is believed to have crashed in 2014 with 239 people on board.
Wattrelos said investigators hoped to travel to the US to meet with the FBI, which examined MH370 Captain Zaharie Shah's home flight simulator and representatives of Boeing in a bid to obtain and re-examine raw data.
A similar trip planned in September 2017 was cancelled after US authorities demanded the signing of "confidentiality clauses" to protect Boeing "industry secrets".
He said French investigators had identified a "third entity" in possession of information and/or data relating to the movements of the missing plane.
"We are a little angry and now we want to say stop, it is time that the United States really cooperate on this issue," Wattrelos said.
"It is necessary to go there because there are three entities that hold important information for understanding what happened on this flight."
In addition to verifying data provided by the FBI and Boeing, investigators were seeking to establish whether the "third entity" sold software capable of reprogramming or even hacking the Satcom, the antenna that communicates to the Inmarsat satellite from the aircraft.
"The essential trail is the Inmarsat data. Either they are wrong or they have been hacked," Wattrelos said.
"However, these satellite data are essential to better understand the trajectory of the aircraft."
The identity of the "third entity" is unclear but in a Facebook message posted over the weekend, Wattrelos refers to SITA.
SITA is a company which supplies Malaysia Airlines with communications via VHF radio and Inmarsat satellites for its fleet's ACARS (Aircraft Communications Addressing and Reporting System) avionics.
In the wake of MH370's disappearance, SITA issued a statement saying it was co-operating with authorities investigating the plane's fate.
"The Malaysia Airlines ACARS avionics communications via the SITA network is proprietary to the airline," it said.
"We are fully supporting the airline and all the relevant authorities in their ongoing investigation of flight MH370."
Engineer and aviation expert Dr Victor Iannello, who was a member of the Independent Group (IG) of advisers assisting the ATSB in the original search for MH370, expressed doubt the French would uncover evidence data had been deliberately removed or hacked.
"It's not clear what additional information the French investigators expect to obtain while in the US. Boeing has co-operated with the Annex 13 investigation team, and is unlikely to provide private French investigators with data that has not already been made public," he wrote on his blog. Meanwhile, the FBI is unlikely to release information on matters related to ongoing or past investigations.
"The mysterious 'third entity' referred to by Mr Wattrelos that might be selling software capable of maliciously altering SATCOM data is also unknown, although there are a handful of companies in the US and Canada that supply hardware and software for designing, building, and testing parts of the Inmarsat network."
Iannello said independent investigators trying to crack the MH370 mystery were "at an impasse".
"Although the overwhelming consensus is that MH370 did indeed crash in the Southern Indian Ocean, the considerable efforts of official and private investigators have not succeeded in locating the debris field on the seabed," he said.
"The data we have, notably the satellite data, is imprecise, so additional data is needed to reconstruct the trajectory of the plane.
"There is always the chance that during Mr Wattrelos' visit to the US, some new evidence or insights will be uncovered that help us to better understand the disappearance and to find the plane.
"More likely, the existence of helpful new information will be found in Malaysia."
France is the only country still actively investigating the fate of the plane, which vanished en route from Kuala Lumpur to Beijing on March 8, 2014 with 239 on board.
A three year, $200 million Australian taxpayer funded search of a previously uncharted, 120,000sq km section of seabed along the 7th Arc failed to uncover any trace of the doomed jet.
A second search carried out earlier this year by deep sea exploration company Ocean Infinity, which was offered a finder's fee of up to $90 million by the Malaysian Government if it found the wreckage within 90 days, also came up empty.