PERTH, Australia - KUALA LUMPUR, Malaysia (CNN) -- A top Malaysian official on Sunday reaffirmed the importance of finding the black boxes from Malaysia Airlines Flight 370, if the mystery of the missing airliner is ultimately to be solved.
For instance, it would be difficult for investigators to clear crew or passengers until the two recorders are located, Malaysia's acting transport minister, Hishammuddin Hussein, said at a news conference in Kuala Lumpur.
The inspector general of police has found nothing suspicious about the passenger manifest, Hishammuddin said, but "he did not say that they all had been cleared on the four issues that the police are still investigating, which is the possible hijacking, issues of terrorism, psychological and personal problems.
"That is an ongoing thing, and I don't think the IGP would have meant that they have all been cleared, because unless we find more information, specifically on data in the black box, I don't think any chief of police would be in the position" to declare the cases cleared, he said.
The plane's senior pilot, Zaharie Ahmad Shah, 53, has received a lot of attention in the media in the wake of the disaster. Investigators believe it was his voice speaking the last words heard from Flight 370, "Good night, Malaysian 370."
In recent years Zaharie was active on social media, posting videos in which he explained how to optimize an air conditioning system to reduce electricity bills and showing photos of his many gadgets. He loved food and cooking.
He was also passionate about politics, urging people to vote out the current Malaysian government. But nothing in his social media posts would seem to suggest foul play in MH370's disappearance.
Still seeking pings
The search continued Sunday, 36 days after the plane carrying 239 people vanished from radar screens early March 8 during a flight from Kuala Lumpur to Beijing.
After a week during which the search area in the southern Indian Ocean shrank each day, it grew significantly on Sunday -- by almost 40%.
The Australian Maritime Safety Authority said teams would conduct a visual search of approximately 22,200 square miles (57,500 square kilometers). The center of the search area lies about 1,370 miles (2,200 kilometers) northwest of Perth, Australia.
Up to 11 military aircraft, one civil aircraft and 14 ships assisted in Sunday's search for the missing airliner, the Australia's Joint Agency Coordination Centre said.
The search area has shifted each day as officials look at new data and study the ocean currents. Still, no debris has been found and promising audio signals heard days ago were far apart.
And there are concerns that the batteries in the data recorders have faded or may have stopped working.
Jeff Wise, an aviation analyst, said Sunday's enlarging search area makes observers wonder.
"It doesn't really inspire confidence (in the people running the search)," he told CNN's "New Day."
He said the fact that searchers haven't heard pings in five days makes him wonder whether it's time to send down underwater vehicles with sidescan sonar.
"Unless they have some other information that we are not aware of, it seems like if they really did think this was the plane that they would go down and really try to locate it," he said.
On Saturday, searchers aboard the Australian vessel Ocean Shield continued towing the ping locator -- referred to as a TPL -- at a walking pace through the water in hopes of picking up new signals from either or both of the locator beacons that were attached to the plane's cockpit voice recorder and flight data recorder, said Cmdr. William Marks, the U.S. Navy commander in charge of the American presence involved in the search effort.
The batteries that send out the signals were certified to last 30 days, a deadline that's already passed.
"We are in a transition period at the moment," retired Lt. Col. Michael Kay of the Royal Air Force told CNN, referring to the fact that searchers will soon have to give up hunting with pinger locators and switch to sonar. "We know that the (data recorder) batteries last between 30 and 40 days."
Once the searchers have concluded that there is no hope that the batteries could still be powering the beacons, searchers will lower into the water a sonar device called the Bluefin-21 to scour the ocean floor. The Bluefin's pace is slower than that of a TPL, Marks said.
Four pings, one dud
On April 5, the towed pinger locator detected two sets of underwater pulses of a frequency close to that used by the locator beacons. Three days later, last Tuesday, it reacquired the signals twice.
All four signals were within 17 miles of one another.
A fifth ping, detected Thursday by a sonobuoy dropped from an airplane, is "unlikely to be related to the aircraft black boxes," Australian chief search coordinator Angus Houston said a day later.
CNN's Nic Robertson reported from Kuala Lumpur and Ralph Ellis wrote from Atlanta. CNN's Steve Almasy, David Molko, Elizabeth Joseph and Sumnima Udas and journalists Ivy Sam and Chan Kok Leong contributed to this report.
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