PERTH, Australia — Search crews in the Indian Ocean failed to pick up more of the faint underwater sounds that may have been from the missing Malaysian jetliner’s black boxes whose batteries are at the end of their life.

The signals first heard late Saturday and early Sunday had sparked hopes of a breakthrough in the search for Flight 370, but Angus Houston, the retired Australian air chief marshal leading the search far off western Australia, said listening equipment on the Ocean Shield ship has picked up no trace of the sounds since then.

Finding the sound again is crucial to narrowing the search area so a submarine can be deployed to chart a potential debris field on the seafloor. If the autonomous sub was used now with the sparse data collected so far, covering all the potential places from which the pings might have come would take many days.

“It’s literally crawling at the bottom of the ocean so it’s going to take a long, long time,” Houston said.

The locator beacons on the black boxes have a battery life of only about a month — and Tuesday marked exactly one month since the plane vanished. Once the beacons blink off, locating the black boxes in such deep water would be an immensely difficult, if not impossible, task.

“There have been no further contacts with any transmission and we need to continue (searching) for several days right up to the point at which there’s absolutely no doubt that the batteries will have expired,” Houston said.

If, by that point, the U.S. Navy towed pinger locator has failed to pick up more signals, the sub will be deployed. If it maps out a debris field on the ocean floor, the sonar system on board will be replaced with a camera unit to photograph any wreckage.

Earlier, Australia’s acting prime minister, Warren Truss, had said the Bluefin 21 autonomous sub would be launched on Tuesday, but a spokesman for Truss said later the conflicting information was a misunderstanding, and Truss acknowledged the sub was not being used immediately.

Houston earlier said the two sounds heard Saturday and Sunday are consistent with the pings from an aircraft’s black boxes.

Defense Minister David Johnston called the sounds the most positive lead and said it was being pursued vigorously. Still, officials warned it could take days to determine whether the sounds were connected to the plane that vanished March 8 on a flight from Kuala Lumpur, Malaysia, to Beijing with 239 on board.

“This is an herculean task — it’s over a very, very wide area, the water is extremely deep,” Johnston said. “We have at least several days of intense action ahead of us.”

Houston also warned of past false leads — such as ships detecting their own signals. Because of that, other ships are being kept away, so as not to add unwanted noise.

“We’re very hopeful we will find further evidence that will confirm the aircraft is in that location,” Houston said. “There’s still a little bit of doubt there, but I’m a lot more optimistic than I was one week ago.”

Such optimism was overshadowed by anguish at a hotel in Beijing where around 300 relatives of the flight’s passengers — most of whom were Chinese — wait for information about the plane’s fate.