Head investigator Kok Soo Chon said, while its unlikely a deliberate diversion by the pilot or first officer, they can not exclude the possibility of a third party interference.
"The answer can only be conclusive if the wreckage is found", he said when asked if they would ever find out what happened on the plane. "It has been established that the air turn-back was done under manual control, not autopilot.we can not rule out unlawful interference by a third party".
"We can not exclude that there was an unlawful interference by a third party", Kok said. "They have to continue the search until they find the plane", she said.
The report, to be released later on Monday, is expected to contain the final details of the investigation into the Malaysia Airlines plane that vanished carrying 239 people, including six Australians, en route from Kuala Lumpur to Beijing in March 2014.
Chief investigator Kok Soo Chon told reporters in Kuala Lumpur that given the plane had been manually diverted and the communications systems turned off by hand, process of elimination meant someone else might have been responsible.
The Malaysian police previously concluded there were no unusual activity on the home simulator.
The report, prepared by a 19-member worldwide team, reiterated Malaysia's assertion the plane was deliberately diverted and flown for over seven hours after severing communications.
Claims that Hamid's mobile phone was used found that it was only a "heat signal", which Kok said "was just a signal heat to show that the phone was turned on, but there was no call".
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It also concluded that all four of MH370's emergency locator transmitters (ELT), malfunctioned, meaning they did not give off the normal distress signals which would help locate the plane.
Kok said the final report was prepared in consultation with seven other countries, including Australia, the US, China, Indonesia, and the United Kingdom, which endorse its findings. He said there was no evidence of abnormal behaviour or stress in the two pilots that could lead them to hijack the plane but all passengers were also cleared by police and had no pilot training.
"The one point they stressed was that this report was not to assign blame, it was only a safety investigation", she said, adding that the investigators were limited in their effort, as it was based on information supplied to them.
Overall the safety investigators found most fault with the air traffic controllers, first in Kuala Lumpur and then Ho Chi Minh City, who were meant to be watching the plane but who had not followed protocol, meaning the plane was off radar for about 20 minutes before anyone was alerted. And a second, private search by U.S. company Ocean Infinity that finished earlier this year also found no sign of the wreckage.
The investigators did issue a criticism of air traffic controllers in Malaysia and Vietnam for not sounding the alarm immediately, meaning search and rescue operations were delayed.
No sign of it was found in a 120,000-square kilometre Indian Ocean search zone and the Australian-led hunt, the largest in aviation history, was suspended in January past year.
According to this theory, captain Zaharie Ahmad Shah deliberately chose to plunge the plane into an area of the ocean up to 25,000 feet deep, where it would be nearly impossible to find.