PERTH, Australia — For the fifth time in recent days, an underwater sensor detected a signal Thursday in the same swath of the southern Indian Ocean, raising hopes that searchers are closing in on what could be a flight recorder from a missing Malaysian jet.

An Australian air force P-3 Orion, which has been dropping sonar buoys into the water near where four sounds were heard earlier, picked up a “possible signal” that may be from a man-made source, said Angus Houston, who is coordinating the search for Flight 370 off Australia’s west coast.

The latest acoustic data would be analyzed, he said. If confirmed, the signal would further narrow the hunt for the Malaysia Airlines Boeing 777, which vanished March 8 while flying from Kuala Lumpur to Beijing with 239 people aboard.

The Australian ship Ocean Shield, which is towing a U.S. Navy device to detect signal beacons from a plane’s flight data and cockpit voice recorders, picked up two underwater sounds Tuesday. Two sounds it detected Saturday were determined to be consistent with the pings emitted from the flight recorders, or “black boxes.”

The searchers are trying to pinpoint the location of the source of the underwater signals so they can send down a robotic submersible to look for wreckage and the flight recorders from the Malaysian jet.

The underwater search zone is currently a 500-square-mile patch of the ocean floor – about the size of the city of Los Angeles – and narrowing the area as much as possible is crucial before the submersible is sent to create a sonar map of a potential debris field on the seabed.

Separately, a Malaysian government official said Thursday that investigators have concluded the pilot spoke the last words to air traffic control, “Good night, Malaysian three-seven-zero,” and that his voice had no signs of duress. A re-examination of the last communication from the cockpit was initiated after authorities last week reversed their initial statement that the co-pilot was speaking different words.

The senior government official spoke on condition of anonymity because he wasn’t authorized to speak to the media. The conclusion was first reported by CNN.