Malaysia's national police chief is warning that authorities may never learn what exactly led to the disappearance of Malaysia Airlines Flight 370.
The comments on Wednesday came 25 days after the Boeing 777 vanished en route from Kuala Lumpur to Beijing, with 239 people on board.
An international search for debris in the southern Indian Ocean and a criminal investigation by Malaysian police have so far proven fruitless.
Inspector General Khalid Abu Bakar said investigators still need more time to search for clues, warning the probe "may go on and on and on."
Khalid said police continue to focus on the possibilities of hijacking, sabotage, and personal or psychological problems of those on board.
He said investigators have conducted 170 interviews and that more statements need to be collected.
Malaysian officials have said they believe someone intentionally diverted the plane before it crashed into the remote and treacherous waters off the northwest coast of Australia.
The search for possible wreckage continued Wednesday, with ten aircraft and nine ships joining the effort. Australian maritime officials said visibility was favorable, unlike in recent days, when bad weather forced the search to be suspended.
Australian Prime Minister Tony Abbott said his country is committed to finding out what happened to the plane.
"It's one of the great mysteries of our time. It's a terrible tragedy. There are 239 devastated families. There are a lot of very concerned people right around the world and Australia is leading the search and recovery effort as is right given that it all happened in our search and rescue zone. We owe it to the world, we owe it to those families to do whatever we reasonably can to get to the bottom of this," said Abbott.
The search is also expanding underwater, with the arrival of the British Royal Navy nuclear submarine HMS Tireless. It is the first submarine to join the mission.
Time is running out to detect the signal from the missing plane's flight recorder, or black box, which is powered by a battery that usually lasts only 30 days.