Investigators said Monday they still do not know why Malaysia's Flight MH370 vanished four years ago in aviation's greatest mystery, sparking anger and disappointment among relatives of those on board.

In a long-awaited report the official investigation team pointed to failings by air traffic controllers, said the course of the Malaysia Airlines plane was changed manually, and refused to rule out that someone other than the pilots had diverted the jet.

But after years of fruitless searching for the Boeing 777 that disappeared in March 2014 with 239 people aboard, the report offered nothing concrete to grieving relatives of passengers and crew hoping for some sort of closure.

"The team is unable to determine the real cause for the disappearance of MH370," concluded the largely technical 400-page report, noting that investigators were hindered in their probe as neither the plane's wreckage nor its black boxes had been found.

Investigators said the plane was airworthy and the pilots were in a fit state to fly and dismissed the theory that the plane had been taken over remotely to foil a hijacking.

Relatives who were briefed at the transport ministry in the administrative capital Putrajaya before the report's public release expressed anger that there was nothing new in the document, with some storming out of the briefing as frustration boiled over.

"It is so disappointing," said Intan Maizura Othman, whose husband was a steward on MH370, which had been flying from Kuala Lumpur to Beijing carrying mostly mainland Chinese passengers when it vanished.

"I am frustrated. There is nothing new in the report."

She said the meeting between relatives and officials descended into a "shouting match" as anger mounted.

G. Subramaniam, who lost a son on the flight, added that "unsatisfactory responses left many angry".

Largest hunt in history

The disappearance of MH370 triggered the largest hunt in aviation history. But no sign of it was found in a 120,000-square kilometre (46,000-square mile) Indian Ocean search zone and the Australian-led hunt was suspended in January last year.

United States (US) exploration firm Ocean Infinity resumed the search in a different location at the start of this year on a "no find, no fee" basis, using high-tech drones to scour the seabed. But that search was also called off after failing to find anything.

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.

Malaysia's new government, which took power in May, has said the hunt could be resumed but only if new evidence comes to light.

Transport Minister Anthony Loke insisted Monday that "the aspiration to locate MH370 has not been abandoned and we remain ever-hopeful that we will be able to find the answers we seek when the credible evidence becomes available".

One area that came in for criticism in the report by the 19-member team, which included foreign investigators, was air traffic control. It said both Malaysian air traffic control and their Vietnamese counterparts failed to act properly when the Boeing jet passed from Malaysian to Vietnamese airspace and disappeared from radars.

Air traffic controllers did not initiate emergency procedures in a timely fashion, delaying the start of the search and rescue operation, it said.

However, it played down concerns about the pilot and first officer, saying neither appeared to have suffered difficulties in their personal lives that could have affected their ability to fly.

"We did not find any change to their behaviour, everything was normal," Kok Soo Chon, head of the investigation team, told a press conference.

The report also said the plane was airworthy and did not have major technical issues, with Kok saying it had been diverted from its intended flight path manually.

Intervention by a third party could not be ruled out, the report said, but also added there was no evidence to suggest the plane was flown by anyone other than the pilots. - AFP