Search crews hunting for the missing Malaysia Airlines?flight MH370?have failed to relocate the electronic pulse signals that officials said were consistent with?aircraft black boxes, the head of the search operation said Tuesday.

Angus Houston, head of the Joint Agency Coordination Center ( JACC), said sound locating equipment on board the Australian vessel "Ocean Shield" has picked up no trace of the signals since they were first heard late Saturday and early Sunday.

Fears the plane's emergency locator batteries have died are growing by as time is running out to find the devices, whose locator beacons have a battery life of about a month. Tuesday marks exactly one month since the plane vanished.

Houston said the Ocean Shield crew may spend several more days towing sophisticated U.S. Navy listening equipment deep within the ocean to try and find the sounds again. Only at that point, Houston said, would a submarine on board the ship be sent below the surface to try and chart out any debris on the sea floor. If it maps out a debris field, the crew will replace the sonar system with a camera unit to photograph any wreckage.

Houston's comments contradicted an earlier statement from Australia's acting prime minister, Warren Truss, who said search crews would be start the Bluefin 21 autonomous submarine on Tuesday.

The towed pinger locator detected late Saturday and early Sunday two distinct, long-lasting sounds underwater that are consistent with the pings from an aircraft's "black boxes" — the flight data and cockpit voice recorders.

Houston dubbed the find a promising lead in the monthlong hunt for clues to the plane's fate, but warned it could take days to determine whether the sounds were connected to Flight 370, which vanished March 8 on a flight from Kuala Lumpur, Malaysia, to Beijing.

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