Investigators released a report on missing Malaysia Airlines flight MH370 on Monday, saying the Boeing 777's controls were likely deliberately manipulated to take it off course but they were not able to determine who was responsible.
They had no conclusion about what happened aboard the plane that vanished with 239 people on board en route to Beijing from Kuala Lumpur on March 8, 2014, leaving one of the world's greatest aviation mysteries unsolved.
"The answer can only be conclusive if the wreckage is found," Kok Soo Chon, head of the MH370 safety investigation team, told reporters.
On May 29, Malaysia called off a three-month search by U.S. firm Ocean Infinity, which spanned 112,000 sq km (43,243 sq miles) in the southern Indian Ocean and ended with no significant findings.
It was the second major search after Australia, China and Malaysia ended a fruitless A$200 million ($147.06 million) search across an area of 120,000 sq km (46,332 sq miles) last year.
Malaysian and international investigators have been looking into why the Boeing 777 jet veered thousands of miles off course from its scheduled route before eventually plunging into the Indian Ocean.
Experts believe someone may have deliberately switched off MH370's transponder before diverting it over the Indian Ocean.
The last communication from the plane was from the Captain Zaharie Ahmad Shah who signed off with "Good night, Malaysian three seven zero", as the plane left the Malaysian airspace.
A 440-page final report by the Australian Transport Safety Bureau (ATSB) last year showed that Zaharie had flown a route on his home flight simulator six weeks earlier that was "initially similar" to the one actually taken by MH370.
A forensic report by the Malaysian police previously concluded that there were no unusual activities other than game-related flight simulations.
Kok said the investigators examined the history of the pilot and the first officer, and they were satisfied with their background and training and mental health.
"We are not of the opinion it could have been an event committed by the pilots," he said, but added they were not ruling out any possibility since the in-air turn back was done manually and the systems in the plane were also manually turned off.
"We cannot exclude that there was an unlawful interference by a third party," Kok said.
He added all the passengers of the 15 countries had their backgrounds checked by their respective countries and all came back with a clean bill of health.
The next-of-kin of the passengers were briefed on the final report by investigators earlier on Monday.
Voice 370, a group representing the relatives, has previously urged the Malaysian government for a review of the flight, including "any possible falsification or elimination of records related to MH370 and its maintenance".
"We hope that these mistakes will not be repeated and that measures are put in place to prevent them in the future," said Grace Nathan, a lawyer whose mother, Anne Daisy, was on the plane.
"The one point they stressed was that this report was not to assign blame, it was only a safety investigation," she said.
Chinese Foreign Ministry spokesman Geng Shuang said Beijing had paid close attention to the MH370 investigation.
"We hope that all sides can continue to remain in close contact and coordination, to properly carry out relevant follow-up work," he told a daily news briefing, without elaborating.
The only confirmed traces of the aircraft have been three wing fragments washed up along the Indian Ocean coasts.
Malaysia's newly elected Prime Minister Mahathir Mohamad has said Malaysia would consider resuming the search for MH370 only if new clues come to light.