"Investigations may go on and on and on. We have to clear every little thing," Inspector-General of Police Khalid Abu Bakar said.
"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 told reporters here.
After intense investigations, some clues into the missing MH370 have been derived after interrogating 170 people, including family members of passengers, pilot and co-pilot, Bakar said, adding that more people will be quizzed.
It was not known how strong the leads were as the official did not elaborate.
"I do not wish to comment on the background checking of the pilot or cabin crew as they are the subjects of investigations. Passengers have been cleared of the four focus areas of probe - personal and psychological problems, sabotage and hijacking," he said.
The Boeing 777-200's cargo of mangosteens has also come under scrutiny besides the food served onboard the airliner that went mysteriously missing on March 8 after its take-off from Kuala Lumpur with 239 people onboard.
Khalid Abu Bakar said the police had to scrutinize carefully to rule out sabotage.
"For example, when we knew there was a load of mangosteens onboard, we had to find out where the mangosteens came from. We tracked down who plucked the fruits, who packed them and shipped them out, and who put them on the plane.     

"Then we had to determine who would have received them in China and who paid for it, and for how much. Imagine how many people we must interview to rule out sabotage and that is just the mangosteens," Khalid said.
Over 170 statements from foreign and local sources as well as the families of passengers and crew aboard MH370 have been recorded.
"We must be very thorough and we need all the time ... you cannot hurry us," he said.
The police are treating the case as a criminal probe. Khalid warned that authorities may never know what caused the disappearance of flight MH370.
The flight's captain, Zaharie Ahmad Shah, 53, and First Officer Fariq Abdul Hamid, 27, have come under close scrutiny as investigators believe the plane was deliberately diverted by someone on board.
Zaharie's flight simulator is being examined and Malaysian authorities were still awaiting feedback.

UK submarine joins plane hunt, Malaysian PM to visit Perth

The prolonged search for the crashed airliner continued on Wednesday as a British nuclear submarine joined the multinational hunt in the Indian Ocean, ahead of Malaysian Prime Minister Najib Razak's visit here.
Razak will visit the RAAF Pearce air base - the departure point for the seven-nation search for the missing Malaysia Airlines Flight MH370, that went mysteriously missing on March 8 after its take-off from Kuala Lumpur with 239 people on board.
According to media reports here, Britain's Royal Navy submarine HMS Tireless arrived in the search area about 1,500 kilometres northwest of Perth this morning. The personal jet of Peter Jackson - Oscar-winning New Zealand movie director - is also being used in the hunt.
Up to 10 planes and nine ships will assist in today's search for the Boeing 777-200, a new Joint Agency Co- ordination Centre (JACC) here managing the search said in a statement today.
The Australian Maritime Safety Authority has determined a search area of about 221,000 square kilometres, 1,504 kilometres northwest of Perth.
Nine military planes will assist in the search, while one civil jet will provide a communications relay. The first aircraft departed for the search area at 6am local time.
Nine ships have been tasked for the search, that entered its 25th day on Wednesday.
The weather forecast for Wednesday's search is for marginal conditions, with areas of broken cloud, sea fog and isolated thunderstorms, reducing visibility, the statement said.
Retired Australian Air Chief Marshal Angus Houston, heading the JACC, on Tuesday said it was the most challenging search and rescue operation he had ever seen and warned efforts to trace the wreckage could take time.
He said the task was "very complex", "very demanding" because the teams had no hard information to work from.


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