Just when it looked as if international teams searching for the missing Malaysia Airlines jet were closing in on their target in the Indian Ocean, an Australian vessel with a high-tech listening device reported a second signal a substantial distance away.
With planes and ships rushing to where search ship from China said it heard an electronic signal that could have come from one of the missing plane's data recorders, Australia's HMAS Ocean Shield reported hearing a similar signal Sunday, but 300 nautical miles away.
The Ocean Shield is equipped with a highly sensitive US Navy detection device designed to pick up signals from black boxes, according to the Reuters international news service.
"We are treating each of them [the reports] seriously," retired Air Chief Marshal Angus Houston, head of Australia's Joint Agency Coordination Centre, told a news conference in Perth, Australia.
"We need to ensure before we leave any of those areas that this does not have any connection with MH370," Houston said.
Officials hope the black boxes, which record flight data and cockpit conversations, will reveal what happened to cause the Malaysia Airlines flight to veer sharply off-course and fly over a vast uninhabited portion of the ocean until it presumably exhausted its fuel and crashed.
But two dozen ships and planes searching for the missing plane for a month have not found any debris or other evidence linked to the Boeing 777.
Flight 370 disappeared Dec. 8 from radar screens less than an hour after taking off from Kuala Lumpur on a routine flight to Beijing.
More than 200 passengers and crew members are missing and presumed lost.
The Chinese patrol ship, the Haixun 01, twice reported hearing signals compatible with what a black box would send out over the weekend.
More than half of the missing passengers are Chinese nationals.
But authorities cautioned that there still was no evidence that the signals, if confirmed, came from the missing plane.
A US government source close to the investigation told Reuters on Sunday that the signals, or "pings," had not yet been validated.
The source also said no additional information had been discovered to explain why the plane disappeared or why it appeared to have crashed thousands of miles from its planned flight course, Reuters said.
Houston said continuing analysis of satellite data has enabled investigators to refine the search area to an area of the southern Indian Ocean about 1,200 miles northwest of Perth.
"The area of the highest probability is, what we think, the southern part where Haixun 01 is operating," Houston said.
"That is why we are really interested in the two acoustic encounters that Haixun 01 has had," he said.
In that part of the ocean, the water is nearly 15,000 feet deep, Reuters said.
Australian Prime Minister Tony Abbott said he was "hopeful" the reported signals were related to the missing plane.
"This is the most difficult search in human history," Abbott said from Tokyo, where is was visiting on Sunday.
"We are searching for an aircraft which is at the bottom of a very deep ocean and it is a very, very wide search area," he said.
Authorities believe the plane was deliberately diverted from its planned course and its communications devices disabled, but have not absolutely ruled out the possibility of mechanical problems.
Malaysia said Saturday it was convening a formal investigation into the plane's disappearance that included experts from Australia, the United States, China, Britain and France.