A top Malaysian official on Sunday reaffirmed the importance of finding the black boxes of Malaysia Airline 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 press conference in Kuala Lumpur.

The Inspector General of Police (IGP) has found nothing suspicious with 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 blackbox, I don't think any chief of police would be in the position" to declare the cases cleared, he said.

Still seeking pings

More than 35 days since the plane vanished from radar screens early March 8 during a flight from Kuala Lumpur, Malaysia, to Beijing, the search continued.

Up to 11 military aircraft, one civil aircraft and 14 ships will assist in Sunday's search for the missing airliner, the Australia's Joint Agency Coordination Centre said. The Australian Maritime Safety Authority planned a visual search area totaling approximately 22,203 square miles (57,506 square kilometers). The center of the search area lies about 1,367 miles (2,200 kilometers) northwest of Perth.

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 signals that were attached to the plane's cockpit voice recorder and flight data recorder, said Cmdr. William Marks, the U.S. Navy commander leading the American effort to find the plane.

The batteries that send out the signals were certified to last 30 days, but the beacon manufacturer predicted they would last days longer.

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 the Bluefin-21, a sonar device, to scour the ocean floor. The Bluefin's pace is slower than that of the TPL, Marks said.

Four pings, one dud

On April 5, the TPL 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.