Malaysia's top police official warned Wednesday that authorities may never learn what caused the mysterious disappearance of flight MH370, as he indicated a three-week-old criminal investigation has so far been inconclusive.

"Give us more time," Inspector General of Police Khalid Abu Bakar told reporters in Kuala Lumpur, according to Dow Jones Newswires.

"We may not even know the real cause of this incident."

The sober assessment is unlikely to go down well with anxious family members of the missing passengers, especially Chinese relatives who have fiercely attacked Malaysia's government and the airline as incompetent "liars" and "murderers".

Two thirds of the 227 passengers were Chinese.

Malaysian police have said they were investigating the backgrounds of all 239 people on board the Malaysia Airlines jet, who included 12 crew members, as well as ground crew and flight engineers.

Their criminal probe has focused on the possibility of a hijacking, sabotage or psychological problems among passengers or crew.

Khalid was quoted by state news agency Bernama as saying that all of the passengers had been "cleared" by Malaysian police, but he indicated the plane's two pilots were still being looked at.

Authorities have earlier said background checks by more than a dozen countries on their nationals on the plane came back clear.

Attention has focused on the flight's captain, Zaharie Ahmad Shah, 53, and First Officer Fariq Abdul Hamid, 27, as Malaysia believes the plane was deliberately diverted by someone on board.

But no evidence has emerged to suggest a motive by either of the men, who appear to have been well-regarded by their peers.

Khalid said police had recorded more than 170 statements so far.

"This investigation is ongoing. There are still more people we need to interview," he said, according to Dow Jones, declining to provide further details while the probe was under way.

The Boeing 777 disappeared on March 8 shortly after take-off on an overnight flight from Kuala Lumpur to Beijing.

Malaysian police and FBI experts were examining a flight simulator assembled by captain Zaharie at his home, hoping to find any clues.

But Khalid said he was still awaiting feedback from experts examining the simulator, adding that so far nothing conclusive had emerged.

A multi-nation search for wreckage from the plane is under way in the Indian Ocean after Malaysia said satellite data indicated it may have gone down there.