The months-long hunt for a missing Malaysian airliner has hit an impasse, with investigators determining Thursday that the jet is not in an area of the Indian Ocean previously deemed its most likely crash site.

Since early April, the search had focused on an area where crews detected four deep-sea acoustic signals — presumably from the black boxes of Malaysia Airlines Flight 370. On Thursday, the Australian agency leading the operation said an underwater search of that zone had ended with “no signs of aircraft debris.”

“The search in the vicinity of the acoustic detections can now be considered complete,” Australia’s Joint Agency Coordination Center said in a news release. In the judgment of investigators, it said, “the area can now be discounted as the final resting place of MH370.”

That determination, coming more than 21 / 2 months after the plane’s disappearance, adds further ambiguity to one of the greatest mysteries in aviation history. With the acoustic signals leading to an apparent dead end, the search enters a more daunting stage — one in which crews will scour a broader area of the Indian Ocean without any fresh leads to pursue.

The search of the seafloor will be on pause, Australia said, and will resume in August after a private company is found to manage expertise and provide equipment. Previously, countries have been using military resources in the search — something they say is untenable in an operation that could last years. The expanded underwater search area is about 23,000 square miles, roughly the size of West Virginia.

Compare the depth of Malaysia Airlines Flight 370 to other milestone depths and distances. (Richard Johnson/The Washington Post)

By comparison, the U.S. Navy’s Bluefin-21 — an unmanned submarine — took more than a month to search an area of 330 square miles. That relatively small search zone was drawn up based on the area in which listening equipment had detected the deep-sea pings.

The pings had seemed like an extraordinary stroke of fortune. Angus Houston, the chief of the multinational search, said in early April that he hoped to find evidence of the plane “in a matter of days.”

Instead, the Bluefin found nothing, leading to doubts about whether the pings indeed came from the airplane’s black boxes. The recorders are equipped with emergency beacons that transmit signals for about a month after a crash.

Michael Dean, the U.S. Navy’s deputy director of ocean engineering, told CNN that authorities now believe the pings came from a source unrelated to the missing Boeing 777. But the Australian government on Thursday did not specifically rule out the pings.

Black boxes generally transmit signals at 37.5 kilohertz, but in this instance, the signals were detected at 33.3 kilohertz. Authorities said at the time of detection that signals can fluctuate based on the age of batteries and ocean conditions.

Apparent doubt about the pings is a “show stopper for the near term,” said David Gallo of the Woods Hole Oceanographic Institution, who co-led the search for an Air France flight that crashed in the Atlantic Ocean in 2009. “There were doubts about those pings since day one,” he wrote in an e-mail.

“And now it’s time to move on. To where?”

There has been no explanation for Flight 370’s disappearance or for why it veered from its flight path shortly after takeoff March 8. The red-eye flight from Kuala Lumpur, Malaysia, was bound for Beijing with 239 passengers and crew members aboard, but the jet lost contact with ground control shortly after leaving Malaysian airspace. Malaysian officials say the communication shutdown was a deliberate act by somebody on board.

The only clues to the plane’s whereabouts have come from complex “handshake” signals it sent to a satellite while airborne. The signals were transmitted about hourly and continued for seven hours. Advanced analysis of the signals led investigators to conclude that the plane probably crashed in the Indian Ocean, off the western coast of Australia.

Earlier this week, the Malaysian government released the raw satellite data, a step requested for weeks by the families of those aboard.

Simon Denyer in Beijing contributed to this report.