One of the world's biggest aviation mysteries remains unsolved after investigators said in a report released Monday they do not know what happened to the Malaysia Airlines plane that disappeared four years ago.
In the long-awaited report, a 19-member international team ruled out many possible scenarios about what could have caused the plane to vanish, but said they could not rule out the possibility of "intervention by a third party."
Kok Soo Chon, head of the MH370 safety investigation team, told reporters in Kuala Lumpur that the real cause for the plane's disappearance could only be determined "if the wreckage is found."
He said investigators examined the history of the pilot and first officer and said neither showed signs of abnormal behavior or stress that could have caused them to hijack the plane.
"We are not of the opinion that it could be an event committed by the pilot," Kok said.
But "we cannot rule out unlawful interference by a third party,'' which could include someone holding the pilots hostage, Kok said. He added that no group has claimed it hijacked the plane and noted that none of the passengers had piloting experience or any known motive for wanting the plane to crash.
"Presumably, it would have to have been a stowaway," said Mary Schiavo, former inspector general of the U.S. Department of Transportation and now a private aviation attorney.
Schiavo told VOA that if every passenger on the plane was who they said they were, none of the ticketed passengers would have been able to fly the plane.
“The report went to great lengths to rule out financial problems, mental problems, and conclusively ruled out the rumor that the pilot in command had practiced this maneuver on his home computer,” she said.
“What this report did was to go through many scenarios, most of which they could rule out. And so they said the one thing they couldn't rule out was a third party," she added.
Family members of the flight's passengers and crew expressed their frustration with the report, saying there were gaps in the investigations and too many unanswered questions.
On March 8, 2014, MH370 disappeared. The Boeing 777 was carrying 239 people on a flight from Kuala Lumpur to Beijing.
The governments of Australia, Malaysia and China suspended the official search in January 2017 after scrutinizing about 119,139 square kilometers of the Indian Ocean floor at a cost of more than $150 million. Officials then concluded that the probable crash site was farther north.
After pressure from the families of the victims, the former Malaysian government struck a deal with Ocean Infinity, a U.S.-based exploration company, to restart the search in January 2017, on the condition it would only be paid if the Boeing 777 or its flight data recorders — the black boxes — were found.
The firm stood to make up to $70 million if successful but did not find any sign of the airliner, despite scouring the seabed with some of the world's most high-tech search equipment.
The latest search for the missing jetliner ended in May when Ocean Infinity announced it had called off its three-month effort to find the plane after searching 112,000-square kilometers.
Ira Mellman contributed to this report.