Malaysian civilian aviation chief Azharuddin Abdul Rahman has said disappearance of Malaysia Airlines flight MH370 remains “a mystery”.
The plane – carrying 239 people - vanished from radar almost three days ago en route to Beijing from Kuala Lumpur.
High jacking has not been ruled out, with two passengers believe to be travelling on stolen passports.
Malaysian armed forced are working with Chinese intelligence agencies to establish who the passengers were.
The sighting of a yellow life raft was investigated on Monday, but was found to be an unrelated object.
Earlier, China said the Malaysian authorities needed to “step up their efforts” to find the missing jet which had more than 150 Chinese nationals on board.
There are now 40 ships and 34 aircraft from nine different nations taking part in the search in the seas off Vietnam and Malaysia.
The wait for information has been agonising for the family members of the missing passengers who are waiting for news at a Beijing hotel.
Flight MH370 left Kuala Lumpur for Beijing at 00:41 local time on Saturday (16:41 GMT on Friday).
But radio contact was lost at 17:30 GMT, somewhere between Malaysia and Vietnam.
Officials say they still have no idea what went wrong.