Story highlights The search is "the most difficult in human history," Australia's Abbott says

"We will not rest until answers are indeed found," Malaysian PM says

Australian ambassador: Focus moves due to searchers "eliminating areas"

Official: All 227 passengers have been cleared of hijacking, psych issues

Many have pointed to lessons learned -- and heeded -- from the disappearance of Malaysia Airlines Flight 370.

Should there be tighter rules about who's in the cockpit? That's happened. Should the Malaysian military have acted more quickly after the airliner went missing nearly four weeks ago? They've launched an investigation. Are there better ways to track commercial aircraft -- especially when, as in this case, its transponder is turned off? An international aviation organization says it will consider "all of the options."

But as much as things might change because of this mystery, it doesn't change the fact that -- for yet another day -- there are 239 families still grieving, still waiting, still venting over authorities' inability to answer what happened to the Boeing 777.

"We want to provide comfort to the families and we will not rest until answers are indeed found," Malaysian Prime Minister Najib Razak said Thursday during a visit to a military base near the Australian city of Perth, which has become the hub for search operations.

Yes, authorities have said communications mysteriously, and seemingly purposefully, cut off shortly into the Beijing-bound flight. Yes, satellite data suggests the aircraft turned back over Malaysia before terminating somewhere in the vast southern Indian Ocean.

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Photos: The search for MH370 Photos: The search for MH370 Two years after Malaysia Airlines Flight 370 went missing, a relative of one of the passengers burns incense in Beijing on March 8, 2016. Flight 370 vanished on March 8, 2014, as it flew from Kuala Lumpur, Malaysia, to Beijing. There were 239 people on board. Hide Caption 1 of 43 Photos: The search for MH370 On July 29, police carry a piece of debris on Reunion Island, a French territory in the Indian Ocean. A week later, authorities confirmed that the debris was from the missing flight. Hide Caption 2 of 43 Photos: The search for MH370 Staff members with the Australian Transport Safety Bureau examine a piece of aircraft debris at their laboratory in Canberra, Australia, on July 20. The flap was found in June by residents on Pemba Island off the coast of Tanzania, and officials had said it was highly likely to have come from Flight 370. Experts at the Australian Transport Safety Bureau, which is heading up the search for the plane, confirmed that the part was indeed from the missing aircraft. Hide Caption 3 of 43 Photos: The search for MH370 In late February, American tourist Blaine Gibson found a piece of plane debris off Mozambique, a discovery that renewed hope of solving the mystery of the missing flight. The piece measured 35 inches by 22 inches. A U.S. official said it was likely the wreckage came from a Boeing 777, which MH370 was. Hide Caption 4 of 43 Photos: The search for MH370 Relatives of the flight's passengers console each other outside the Malaysia Airlines office in Subang, Malaysia, on February 12, 2015. Protesters had demanded that the airline withdraw the statement that all 239 people aboard the plane were dead. Hide Caption 5 of 43 Photos: The search for MH370 A police officer watches a couple cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The couple's son was on the plane. Hide Caption 6 of 43 Photos: The search for MH370 Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27, 2014. Data from communications between satellites and the missing flight was released the day before, more than two months after relatives of passengers said they requested it be made public. Hide Caption 7 of 43 Photos: The search for MH370 Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14, 2014. Hide Caption 8 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13, 2014. Hide Caption 9 of 43 Photos: The search for MH370 The HMS Echo, a vessel with the British Roya; Navy, moves through the waters of the southern Indian Ocean on April 12, 2014. Hide Caption 10 of 43 Photos: The search for MH370 A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9, 2014. Hide Caption 11 of 43 Photos: The search for MH370 A relative of a missing passenger cries at a vigil in Beijing on April 8, 2014. Hide Caption 12 of 43 Photos: The search for MH370 Australian Defense Force divers scan the water for debris in the southern Indian Ocean on April 7, 2014. Hide Caption 13 of 43 Photos: The search for MH370 A towed pinger locator is readied to be deployed off the deck of the Australian vessel Ocean Shield on April 7, 2014. Hide Caption 14 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4, 2014. Hide Caption 15 of 43 Photos: The search for MH370 On March 30, 2014, a woman in Kuala Lumpur prepares for an event in honor of those aboard Flight 370. Hide Caption 16 of 43 Photos: The search for MH370 The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, 2014, after other relatives left en masse to protest the Malaysian government's response to their questions. Hide Caption 17 of 43 Photos: The search for MH370 A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27, 2014. Hide Caption 18 of 43 Photos: The search for MH370 Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27, 2014. Hide Caption 19 of 43 Photos: The search for MH370 People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27, 2014. Hide Caption 20 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24, 2014. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived." Hide Caption 21 of 43 Photos: The search for MH370 Grieving relatives of missing passengers leave a hotel in Beijing on March 24, 2014. Hide Caption 22 of 43 Photos: The search for MH370 A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22, 2014. Hide Caption 23 of 43 Photos: The search for MH370 A Chinese satellite captured this image, released on March 22, 2014, of a floating object in the Indian Ocean, according to China's State Administration of Science. It was a possible lead in the search for the missing plane. Surveillance planes were looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia. Hide Caption 24 of 43 Photos: The search for MH370 Satellite imagery provided by the Australian Maritime Safety Authority on March 20, 2014, showed debris in the southern Indian Ocean that could have been from Flight 370. The announcement by Australian officials raised hopes of a breakthrough in the frustrating search. Hide Caption 25 of 43 Photos: The search for MH370 Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight. Hide Caption 26 of 43 Photos: The search for MH370 A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19, 2014. Hide Caption 27 of 43 Photos: The search for MH370 On March 18, 2014, a relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet. Hide Caption 28 of 43 Photos: The search for MH370 U.S. Navy crew members assist in search-and-rescue operations in the Indian Ocean on March 16, 2014. Hide Caption 29 of 43 Photos: The search for MH370 Members of the Chinese navy continue search operations on March 13, 2014. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, search efforts expanded west into the Indian Ocean. Hide Caption 30 of 43 Photos: The search for MH370 A Vietnamese military official looks out an aircraft window during search operations March 13, 2014. Hide Caption 31 of 43 Photos: The search for MH370 Malaysian air force members look for debris near Kuala Lumpur on March 13, 2014. Hide Caption 32 of 43 Photos: The search for MH370 Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12, 2014. Hide Caption 33 of 43 Photos: The search for MH370 A member of the Vietnamese air force checks a map while searching for the missing plane on March 11, 2014. Hide Caption 34 of 43 Photos: The search for MH370 A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported on March 8, 2014. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10, 2014. Hide Caption 35 of 43 Photos: The search for MH370 A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews on March 9, 2014, before returning to search for the missing plane in the Gulf of Thailand. Hide Caption 36 of 43 Photos: The search for MH370 Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9, 2014. Hide Caption 37 of 43 Photos: The search for MH370 Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9, 2014. Hide Caption 38 of 43 Photos: The search for MH370 The rescue vessel sets out from Sanya in the South China Sea on March 9, 2014. Hide Caption 39 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8, 2014. Hide Caption 40 of 43 Photos: The search for MH370 A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8, 2014. Hide Caption 41 of 43 Photos: The search for MH370 Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8, 2014. Hide Caption 42 of 43 Photos: The search for MH370 Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference at a hotel in Sepang on March 8, 2014. "We deeply regret that we have lost all contacts" with the jet, he said. Hide Caption 43 of 43

Yet there have been no solid leads about why any of this happened or where the plane ended up. In fact, officials don't seem to know all that much more on Day 27 after it disappeared sometime after leaving Kuala Lumpur than they did on Day 1.

Australian Prime Minister Tony Abbott on Thursday described the search for the plane as "the most difficult in human history" and warned there was no guarantee it would be found.

"We cannot be certain of ultimate success in the search for MH370," he said at a news briefing in Perth, standing alongside Najib. "But we can be certain that we will spare no effort -- that we will not rest -- until we have done everything we humanly can."

All 227 passengers have been cleared of any role in hijacking or sabotage or having psychological or personal issues that might have played a role in the plane's disappearance, the inspector general of Malaysian police, Khalid Abu Bakar, told reporters Wednesday.

Police said Wednesday a review of a flight simulator found in a pilot's house proved inconclusive. And senior Malaysian government officials told CNN last week that authorities have found nothing about either of the pilots to suggest a motive. There have been no such public comments about the other 10 crew members, however.

"We don't have enough evidence to take (hijacking, sabotage or many other possibilities) off the table," Michael Kay, a former British pilot and military officer, told CNN. "What we need to do is keep an open mind, look at the facts, and keep building the jigsaw puzzle. Because that's all we have at the moment."

On Thursday, up to eight aircraft will set out looking for telltale debris across an 91,500 square-mile (237,000 square-kilometer) zone, the Australian Maritime Safety Authority says. Up to nine ships will search, including one British submarine racing against time to hear the signature pinging from Flight 370's flight data recorder.

There could be a breakthrough imminently; a seat cushion, a soda can, a life jacket could be spotted and scooped up, leading to the rest of the plane and, ultimately, to explain what happened.

So far, though, nothing has been found. In fact, it might be someone dipping her toes off a beach in Australia or Thailand or Malaysia who first notices something.

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"We'll keep going til hell freezes over," Kim Beazley, Australia's former defense minister and current ambassador to the United States, told CNN's Wolf Blitzer. "It could take months, it could take years."

Official pleads for patience

Last Friday, officials announced that -- based on new analysis of satellite data -- they'd shifted the search area significantly closer to Australia's northwest coast. It moved again from Tuesday to Wednesday, albeit not as extensively. On Thursday, it moved again, a little farther north.

Why? Beazley explained it's because "we're eliminating areas from our inquiry" and moving to adjacent areas.

Yet David Soucie, a CNN safety analyst and author of "Why Planes Crash," said "from the outside looking in, it just doesn't seem to make a lot of sense."

"It just looks like they're following information and data that they're not confident in," Soucie said.

Authorities have been upfront about many things they don't know about Flight 370 -- things like altitude, speed and direction that are key to pinpointing its final resting place.

Then there are questions about who and what was aboard.

While the passengers were cleared, investigators are still questioning relatives of all of those on the plane -- having already interviewed about 170 people -- as well as those who may have had access to it.

That includes scrutinizing those who prepared food for the flight, those who packed the cargo, and those who were to receive the cargo in China.

"Everything from beginning to end," said Khalid, the Malaysian police official.

Police are considering four criminal possibilities: hijacking, sabotage, personal problems and psychological issues, though mechanical failure hasn't been ruled out. Khalid stressed getting answers won't be easy, nor will it be quick.

"We have to clear every little thing," said Khalid. "You cannot hurry us in whatever we are doing."

'We cannot let another aircraft simply disappear'

Even without anything definitive, Malaysia Airlines has taken proactive steps to beef up cockpit security, two sources familiar with the airline's operations told CNN on Wednesday.

A new directive says no pilot or first officer is allowed to sit alone in the cockpit. Whenever one of them is outside the cockpit, a senior cabin steward must remain inside.

"These changes are positive in nature and directly relate to the MH370 incident," one of the sources told CNN.

This policy isn't new everywhere: It's long been in place for the United States, experts note.

"What they put in place is pretty common sense," said Mary Schiavo, a CNN aviation analyst and former inspector general for the U.S. Department of Transportation.

Other reforms could be coming as well, and not just affecting Malaysia Airlines.

Tony Tyler, director general and CEO of the International Air Transport Association, in a recent speech pointed to "disbelief both that an aircraft could simply disappear and that the 'black box' is so difficult to recover."

He added, "We cannot let another aircraft simply disappear."

Toward that end, the association will form a task force that will include participation by the International Civil Aviation Organization to "examine all of the options available for tracking commercial aircraft" and then to report its conclusions by December.

A second lesson concerns security, Tyler said, citing the fact two passengers with phony passports were able to board the missing jetliner unchallenged.

Though these two men have since been cleared, the incident underscores a need for governments to do better checking passenger lists, he said. "Airlines are neither border guards nor policemen; that is the well-established responsibility of governments."

Family members still have questions

Such changes are all well and good. But they won't bring back the scores thought to have perished on Flight 370.

On Wednesday, families of 18 Chinese passengers -- out of 154 Chinese aboard -- met privately in Kuala Lumpur for three hours with Malaysian government officials and investigators. The meeting had been called after they accused Malaysia of not being upfront about the investigation.

Azharuddin Abdul Rahman, head of Malaysia's civil aviation department, said it was "a very good meeting." "We answered all their questions."

The families' representative saw it differently. "I personally believe today's meeting had some progress, but the time was short and family members didn't have an opportunity to raise questions," said Jiang Hui.

Questions, questions, questions. There's no shortage of them.

Chief among them is will the flight data recorder be found before batteries on its locator beacon die -- which, according to its design standards, would happen April 7. Experts agree that it won't be easy, given all the unknowns.

"They are looking in a vast area in very deep waters ... and we really have no idea where it went in," said Bill Schofield, an Australian scientist who helped create the flight data recorders that, if found, could prove key to the investigation.

"A needle in a haystack would be much easier to find."