Malaysia has warned that the reasons for the Malaysia Airlines plane's disappearance may never be known, as Prime Minister Najib Razak heads to Australia for talks on the search.
Malaysia's police chief said that their investigation could "go on and on".
Ten planes and nine ships are searching the southern Indian Ocean. A UK submarine has also joined the hunt.
Flight MH370 disappeared on 8 March as it was travelling from Kuala Lumpur to Beijing. It was carrying 239 people.
Mr Najib will arrive in Perth, Western Australia, on Wednesday evening. He will visit the new Joint Agency Coordination Centre (JACC), where the southern Indian Ocean search is being led.
He will meet Australian Prime Minister Tony Abbott, and retired air chief marshal Angus Houston, who is leading the JACC.
Meanwhile, Malaysian police chief Khalid Abu Bakar said the criminal investigation could "go on and on and on. We have to clear every little thing."
"At the end of the investigations, we may not even know the real cause. We may not even know the reason for this incident," he said.
He added that police had "cleared" all the passengers of the four key areas being investigated: hijacking, sabotage, and psychological and personal problems, Malaysia's Bernama news agency reported.
Khalid Abu Bakar added that more than 170 interviews had been conducted with family members of the pilots and crew members, and that even cargo and food served on the plane were being investigated in case of sabotage.
Wednesday's search area is around 221,000 sq km (85,300 sq miles), but cloudy conditions, "sea fog and isolated thunderstorms" will reduce visibility for search planes, JACC said in a statement.
The British submarine HMS Tireless has also arrived in the southern Indian Ocean. It will soon be joined by Royal Navy ship HMS Echo.
The private jet of film director Peter Jackson has also joined the search.
On Tuesday, ACM Houston said it was the most challenging operation he had ever seen, and warned that search efforts "could drag on for a long time".
"The last known position [of the plane] was a long, long way from where the aircraft appears to have gone," complicating the task, he said.
Several floating objects have been found in recent days, but none is believed to belong to the missing plane.
Also on Tuesday, Malaysian authorities released the full transcript of communications between flight MH370 and Kuala Lumpur's air traffic control.
They said there was no indication of anything abnormal in the transcript, although the last words received by ground controllers are different from those previously stated.
Officials say that based on satellite data they have concluded that flight MH370 crashed into the southern Indian Ocean, but many relatives of those on board have demanded proof and expressed anger at what they perceive as a lack of information.
A closed-door briefing is being held in Kuala Lumpur for families of those on the flight.