Malaysia had signed a "no find, no fee" deal with Ocean Infinity to resume the hunt for the plane after the official search led by Australia, Malaysia and China was called off early a year ago.
No sign of the jet was found in a 120,000-sq km (46,000-square mile) Indian Ocean search zone and the Australian-led hunt, the largest in aviation history, was suspended in January previous year.
He added: "I'm not ruling out anything, but there were two psychiatrists in my team and they were responsible for examining the audio recordings of the pilot and they concluded there was no anxiety and no stress in the recording, it was just normal, and they also recorded the footage from CCTV ... they didn't find any significant behavioural changes".
Grieving families who had advanced sight of the report before its official release this morning said it provided no new answers, according to Reuters news agency.
Despite the full wreckage never being found, damage examination indicates the plane was "not configured to land", suggesting no-one was in control of the jet during its final moments.
Many conspiracists have argued that Captain Zaharie Ahmad Shah (or another pilot) was conscious at the end of the flight and that he may have deliberately orchestrated a mass murder suicide.
Rather than providing answers, the 1,500 page report disproved numerous theories that have abounded about what happened to MH370.
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.
Lead investigator Kok Soo Chon said: "What we have established is that the aircraft has turned back".
"The turn back was made not under autopilot but under manual control".
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He said finding the wreckage was crucial to helping them have closure.
Only three confirmed fragments of MH370 have been found, all of them on western Indian Ocean shores, including a two-metre wing part known as a flaperon.
Journalist Zan Azlee has the latest from Kuala Lumpur.
Aircraft communications had purposely been disabled earlier in the flight and although a signal was received, it did not contain information about where it was.
'They need to keep searching - that's a given.
Federal Bureau of Investigation and Interpol flew in to help and 40 ships worked around the clock, with 34 aircrafts flying during daylight hours.
The search resumed in October 2015 but the investigation was constantly hampered by bad weather.
Theories include the suggestion that the whole plane, including the captain, fell unconscious and the plane ran on auto-pilot before crashing into the sea.
Several theories have emerged about how the plane disappeared, with some suggesting the plane was hijacked and others believing someone on board may have deliberately turned off the plane's transponder before diverting it over the Indian Ocean.
There has been speculation Ocean Infinity could resume the search as it remains in waters off the West Australian coast doing work for Woodside Energy.