LONDON — The anguish of families of those on board Malaysia Airlines Flight MH370 was prolonged Monday as an official accident report offered no new findings to explain the disappearance.

Investigators said the lack of debris from the doomed flight made it impossible to reach any conclusions about what happened to the Boeing 777 at the center of one of the world’s greatest aviation mysteries.

"The team is unable to determine the real cause for disappearance of MH370," Kok Soo Chon, head of the investigation team, told a news conference in Malaysia. "The answer can only be conclusive if the wreckage is found.”

The jet disappeared on its way from Kuala Lumpur to Beijing on March 8, 2014 with 239 people on board.

The safety report — required by the International Civil Aviation Organization after every accident — could not determine why the aircraft’s transponder stopped transmitting location information, nor whether it broke up in mid-air or as it hit the southern Indian Ocean.

Investigators repeated the original assertion of Malaysia’s government that the plane was deliberately diverted and flown for over seven hours after severing communications, but could not determine “whether the aircraft was flown by anyone other than the pilots.”

“The possibility of intervention by a third party cannot be excluded,” the 495-page report said.

A slow response by air traffic controllers in Malaysia and Vietnam delayed the launch of search and rescue operations, it said, while the battery in the plane’s emergency locator beacon had expired.

The 495-page MH370 safety report. Sadiq Asyraf / Reuters

The report called for modern passenger jets to be globally trackable. “In this technological epoch, the international aviation community needs to provide assurance to the travelling public that the location of current-generation commercial aircraft is always known,” it said. “It is unacceptable to do otherwise.”

The next-of-kin of the passengers were briefed on the report moments before Monday's news conference.

Grace Nathan, a lawyer whose mother, Anne Daisy, was on the plane, told reporters: "We hope that these mistakes will not be repeated and that measures are put in place to prevent them in the future.”

She added: “Four years on, we are none the wiser.”

A 52-day surface search for MH370 covered an area of several million square miles in the Indian Ocean west of Australia, followed by an underwater search that mapped 274,000 square miles of seabed at depths of up to 20,000 feet. They were the largest aviation searches of their kind in history.

Despite other attempts to locate debris using satellite imagery and the mapping of ocean drifts, the 1,046-day search was called off on Jan. 17, 2017.

Kok Soo Chon said Monday's document is was “not a final report” since the majority of wreckage hasn't been found, and that search efforts were a separate process from the safety investigation.