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Frustrations mounted Tuesday as the mystery of Malaysia Airlines Flight 370 stretched into a second month with no recovery of debris or the black boxes authorities hope will explain the jet's stunning disappearance.

Search crews probing a vast swath of the Indian Ocean for electronic pings that could emanate from those boxes have heard only silence since early Sunday, when the Australian ship Ocean Shield last picked up signals consistent with black box technology.

Tuesday's search effort wrapped up with no report of additional signals.

"That the Ocean Shield has not reacquired the ping is a genuine setback," Peter Goelz, former managing director of the National Transportation Safety Board, told CNN.com. "We're talking now (about) an investigation that could go on months, if not years."

During a press conference Tuesday in Perth, Australia, Minister of Defense David Johnston said crews were prepared to deploy a submersible vessel as soon as they receive another ping transmission.

"This is the most positive lead we've had and as I've said, we are pursuing it aggressively," Johnston said of the pings detected Saturday and Sunday.

"The connections two days ago were obviously a time of great hope that there had been a significant breakthrough and it was disappointing that we were unable to repeat that experience," said Warren Truss, Australia's acting prime minister.

The underwater vehicle, the Bluefin 21 unmanned sub, is loaded with equipment that can create a sonar map of an area to chart any debris on the sea floor. But it works slowly, so the search area must be sharply trimmed first. Authorities have narrowed the search, but it remains focused on an area nearly 30,000 square miles in size.

The intense air and sea search on Tuesday included 14 planes and 14 ships operating in good weather conditions off Australia's western coast, search coordinators said in a statement issued Tuesday in Perth.

Angus Houston, who is heading the search off Australia's coast, has warned that it could be days before authorities determine whether the signals came from the cockpit voice and data recorders on board Flight 370, which was lost early March 8 after departing Kuala Lumpur, Malaysia, with 239 people aboard bound for Beijing.

The Ocean Shield picked up the signals using a U.S. Navy device called a towed pinger locator, which is pulled behind the ship at a depth of about 2 miles.

The pinger locator is designed to detect signals at a range of 1 mile, meaning it would need to be almost on top of the black boxes to detect them if they were on the ocean floor.

The Ocean Shield is continuing to comb the ocean, trying to find the signal again.

Adding to the struggle, a Chinese search vessel, Haixun 01, also said it briefly heard signals over the weekend. Those signals, more than 300 miles from the other signals, were being investigated by a British naval vessel, HMS Echo, which also has sophisticated listening technology.

Contributing: The Associated Press

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