The painstaking search to find Malaysia Airlines Flight 370 got a vote of confidence Friday that the effort is headed in the right direction.
"We have very much narrowed down the search area, and we are very confident that the signals that we are detecting are from the black box on MH370," Australian Prime Minister Tony Abbott told reporters during an official visit to China.
Black boxes are the plane's flight data recorder and the cockpit voice recorder. Locator beacons attached to them are designed to emit high-pitched signals, or pings.
Over the past week, four such pings have been detected by a ping locator towed by the Australian vessel Ocean Shield.
A fifth ping, detected Thursday by a sonobuoy dropped by an airplane, is "unlikely to be related to the aircraft black boxes," Australian chief search coordinator Angus Houston said Friday.
"On the information I have available to me, there has been no major breakthrough in the search for MH370," Houston said in a prepared statement. "Further analysis continues to be undertaken by Australian Joint Acoustic Analysis Centre."
Friday is Day 35 in the search, and the batteries powering the flight data recorders' locator beacons are certified to emit signals for only 30 days after they get wet.
That has injected the search effort with a heightened sense of urgency.
The signal is "starting to fade, and we are hoping to get as much information as we can before the signal finally expires," Abbott said.
A senior Malaysian government official and another source involved in the investigation divulged new details about the flight to CNN on Thursday, including information about what radar detected, the last words from the cockpit and how high the plane was flying after it went off the grid.
Malaysia Airlines Flight 370 disappeared from military radar for about 120 nautical miles after it crossed back over the Malay Peninsula, sources said. Based on available data, this means the plane must have dipped in altitude to between 4,000 and 5,000 feet, sources said.
The dip could have been programmed into the computers controlling the plane as an emergency maneuver, said aviation expert David Soucie.
"The real issue here is it looks like -- more and more -- somebody in the cockpit was directing this plane and directing it away from land," said Peter Goelz, a CNN aviation analyst and former National Transportation Safety Board managing director.
"And it looks as though they were doing it to avoid any kind of detection."
But former U.S. Department of Transportation Inspector General Mary Schiavo was not convinced. She said the reported dip could have occurred in response to a loss of pressure, to reach a level where pressurization was not needed and those aboard the plane would have been able to breathe without oxygen, or to get out of the way of commercial traffic, which typically flies at higher altitudes.
That would have been necessary had the plane's transponder been turned off and it lost communications.
"If you don't have any communications, you need to get out of other traffic," Schiavo said.
"We still don't have any motive and any evidence of a crime yet," she said, adding that most radar can track planes at altitudes below 4,000 feet, so the plane's descent may not have indicated any attempt by whoever was controlling it to hide.
She held out hope that the black boxes hold the answers and that they will be found soon.
New flight details revealed
Malaysian sources told CNN that Flight 370's pilot, Capt. Zaharie Ahmad Shah, was the last person on the jet to speak to air traffic controllers, telling them "Good night, Malaysian three-seven-zero."
The sources said there was nothing unusual about his voice, which conveyed no indication that he was under stress.
One of the sources, an official involved in the investigation, told CNN that police played the recording to five other Malaysia Airlines pilots who knew the pilot and co-pilot.
"There were no third-party voices," the source said.
Imagining the search underwater
Search area shrinks
Up to 12 military aircraft, three civil aircraft and 13 ships were assigned to assist in Friday's search for the Boeing 777-200ER, which was carrying 239 people when it vanished on March 8 on a fight from Kuala Lumpur, Malaysia, to Beijing.
There were no sightings reported by search aircraft or objects recovered by ships Thursday, the Joint Agency Coordination Centre said.
Friday's search area was about 18,000 square miles (46,600 square kilometers), centered 1,436 miles (2,311 kilometers) northwest of Perth.
That's far smaller than the search area's size a few weeks ago.
"It's pretty incredible if you look at where we started, which was virtually the entire Indian Ocean. Now getting it down to what's essentially a couple hundred square miles (where the pings have been detected) is pretty miraculous," U.S. Navy Cmdr. William Marks said.
The Ocean Shield first picked up two sets of underwater pulses Saturday that were of a frequency close to that used by the locator beacons. It heard nothing more until Tuesday, when it reacquired the signals twice. The four signals were within 17 miles of one another.
As the search continues, a U.S. Navy ship will help provide supplies and fuel to the ships that are looking for the missing plane.
The USNS Cesar Chavez will help supply Australian naval ships involved in the search "in the coming days," the Navy said in a statement.
That's probably a sign that search teams are preparing for a lengthy hunt, analysts said.
Tracking pings is only one early step in the hunt to find the plane's data records, wreckage and the people aboard.
"I think they're getting ready for the long haul," said Goelz, the aviation analyst. "Even if they do get four or five more pings, once they drop the side-scanning sonar device down, that is going to be painstaking and long. So I think they are settling in for the long search."