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A new theory being investigated is that the food was poisoned on missing Malaysia Airlines flight MH370.
Authorities are probing every lead possible in the hunt to find out what happened to the missing flight, including scrutinising the food and cargo on the plane.
Malaysia’s top police officer Khalid Abu Bakar said that just investigating a load of mangosteens, a leathery-shelled, tropical Asian fruit, proved to be a difficult exercise.
“For example when we knew there was a load of mangosteens on board we had to find out where the mangosteens came from,” he said. “We tracked down who plucked the fruits, who packed them and shipped them out, who put them on the plane.”
“Imagine how many people we must interview and that was just the mangosteens,” he said.
Investigators had to also track down who was buying the fruit to make sure all leads were covered.
"Investigations may go on and on and on. We have to clear every little thing," he 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."
Malaysia PM visits Perth
Malaysian Prime Minister Najib Razak has been told Australia is doing everything it can to find the missing flight, which disappeared almost four weeks ago with 239 people on board.
Mr Najib arrived at Perth's RAAF Pearce air base, which is the departure point for the eight-nation operation, to meet those involved in the search and tour the facilities.
"I want to assure you Australia is doing everything it can," retired air marshal Angus Houston, who is leading the new Joint Agency Coordination Centre for the search, said.
He said it was one of the most demanding search and rescue operations he'd ever seen, and one of the most complex.
He offered his sympathies to the families of those on board MH370, which disappeared on March 8 while flying from Kuala Lumpur to Beijing.
"I'd like to personally extend my personal condolences to the people of Malaysia," he said.
The former head of Australia's defence forces said the search, co-ordinated by the Australian Maritime Safety Authority and the Australian Transport Safety Bureau, was being reinforced by strong liaison at all levels.
"Prime Minister Najib, today you will see in action the remarkable co-operation occurring between the military crews from across our region and indeed the world, who are contributing to the search effort," he said.
"They are working seamlessly and effectively with our civilian agencies, the Australian Maritime Safety Authority and the Australian Transport Safety Bureau.
"It is a truly international effort and the search, I think, is a powerful example of international co-operation at its very best."
He said Malaysia and Australia were longstanding friends who worked well together.
"To use the Australian term, we're good mates," air chief marshal Houston said.
Mr Najib was accompanied by Prime Minister Tony Abbott.
The initial search focused on the South China Sea, where air traffic controllers lost contact with MH370.
After subsequent searches in the Strait of Malacca, Andaman Sea and northern Indian Ocean, the focus turned to the southern Indian Ocean, based on satellite data.
Hundreds of objects have been spotted in the remote search area, but none have been verified as being part of the plane's wreckage.
The Joint Agency Coordination Centre, established this week in central Perth and staffed by 20 people, said eight planes and nine ships would on Thursday search an area of 223,000 square kilometres, 1680 kilometres west-northwest of Perth.
The first plane left the Pearce air base at 6am (WST).
The weather forecast is fair with visibility of about 10km, but the southern area may experience some isolated showers.
Tweet causes stir
Malaysian defence and acting transport minister Hishammuddin Hussein has come under fire for appearing to agree with a journalist that missing flight MH370 was a "blessing in disguise".
A journalist based in Kuala Lumpur, Ismail Amsyar , tweeted "#MH370 is a blessing in disguise for all of us. I understand now the beauty of unity, the sweetness of having each other. @HishammuddinH2O".
Shortly later, Hishammudin replied with "Right u are:) (sic)".
Users immediately responded, blasting both the journalist and the minister for suggesting that there could be a positive to the plight of the missing Malaysia Airlines plane.
Alan Cook tweeted: “@HishammuddinH2O @IsmailAmsyar hmmmm not sure the families will be happy to hear that translate it so they can read your statment #mh370”.
Hj Azman HMZ also quickly responded: “@HishammuddinH2O @IsmailAmsyarSorry. I beg to differ with max consideratn & respect to the families. MH370 can’t be a blessing!”.
The journalist, who works for The Malaysian National News Agency, responded to the criticism, saying the unity and resilience of the Malaysian people was behind his comments.
“but u shud see how united we are defending our country from being accused, how strong the local media dispelling -ve reports,” he wrote.
He also responded directly to users who had taken aim at him.
“@hjazman got what u mean..but m not being insensitive or anything. I look at them as my family and i want them back. My apology,” he tweeted.
“@hjazman I’ve been covering this issue since day 1. they are all my family, my Malaysian family. if they hurt, so do i,” he followed up with shortly later.
Submarine sent to aid search
A British nuclear sub with underwater search capabilities reinforced the Indian Ocean hunt for flight MH370 on Wednesday, but Malaysian police warned the cause of the jet's mysterious disappearance may never be known.
Despite extensive scouring of the remote waters southwest of Perth where Malaysia believes the jet went down, ships and planes have found no debris that would indicate a crash site.
Malaysian police chief Khalid Abu Bakar said a three-week-old criminal investigation into what caused the flight with 239 people aboard to veer far from its intended route between Kuala Lumpur and Beijing had so far been inconclusive.
The sober assessment will add to the frustrations of anxious family members of the missing passengers, who have denounced Malaysia's handling of the crisis.
"Give us more time," Khalid told reporters in Kuala Lumpur, according to Dow Jones Newswires. "We may not even know the real cause of this incident."
Ten planes and nine ships resumed the hunt Wednesday although authorities warned clouds, fog and isolated thunderstorms would reduce visibility.
Britain's Royal Navy said the first submarine in the operation -- HMS Tireless -- has arrived in the area and "with her advanced underwater search capability will be able to contribute to the attempts to locate the missing plane".
Britain's HMS Echo is also due in the search zone shortly to assist Australia's Ocean Shield naval vessel, which is fitted with a US-supplied "black box" detector and is expected to arrive on Friday.
Black box deadline looms
Malaysian news agency Bernama quoted police chief Khalid saying all MH370 passengers had been "cleared" of suspicion.
Police also are investigating the backgrounds of the plane's 12 crew, as well as ground staff and flight engineers, for signs of a hijack or sabotage plot.
More than three weeks after the flight's March 8 disappearance, authorities remain baffled as to how and why it happened, and warn that unless the black box is found, the mystery may never be solved.
The battery-powered signal from MH370's black box -- which records flight data and cockpit voice communications that could hold clues in the mystery -- usually lasts only about 30 days, with time fast running out to find it.
Malaysian leader heads to Perth
Australia has warned against expectations of quick success in the difficult task of recovering the black box from the deep and vast seas.
Retired Australian air chief marshal Angus Houston, who is heading a new coordination centre in Perth, reiterated Wednesday that the odds were stacked against them.
"The reality is it's the most complex and challenging search and recovery operation I've ever seen," he told national radio.
Malaysian Prime Minister Najib is due to arrive late Wednesday in Perth.
On Thursday, he will tour the air base being used as a staging post for the search and meet with his Australian counterpart Tony Abbott.
Malaysia's handling of the crisis, marked by piecemeal and occasionally contradictory information, has been widely questioned, especially by distraught relatives of the 153 Chinese nationals aboard.
Many have alleged incompetence or even a cover-up by Malaysia, straining ties between the two countries, although the rhetoric has eased in recent days.
In a briefing on Wednesday, Malaysian officials sought to explain to skeptical relatives their conclusion that the plane went down in the Indian Ocean.
That determination, based on complex satellite data, has outraged some families who say wreckage must first be recovered.
The closed-door briefing outside Kuala Lumpur included technical experts from China and Malaysia, a government statement said, and was telecast to other relatives in Beijing.
"Throughout the process, I want to assure you that we have done everything in our power to locate MH370. We have spared no effort and expense," civil aviation chief Azharuddin Abdul Rahman told relatives in his opening statement.
But Steven Wang, a relative in Beijing, said the briefing did not convince him.
"I don't think that anything was explained to us... There are all kinds of possibilities," he said.
On Tuesday, Malaysia Airlines revealed the full radio communications with the pilots of MH370, but the routine exchanges contained nothing unusual and shed no light on the theory of possible pilot involvement.The conversations with air traffic controllers had been the subject of much speculation as suspicions have focused on whether one or both of the pilots deliberately diverted the plane.