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Ships scoured the Indian Ocean on Friday for the faint black-box signals of Malaysia Airlines Flight 370, but they ran into a problem: Whales and other sea life may be interfering with the search.

Two ships, one using special equipment from the U.S. Navy, searched a 150-mile expanse of ocean, hoping to pick up sonic pings from the jet’s data recorders. Experts say those pings will stop any day now, further complicating the search.

Australian authorities said one possible alert was detected but was quickly discounted.

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“False alerts may be experienced from biological sources such as whales or interference from shipping noise,” the Australian Joint Agency Coordination Center said.

An Australian ship, the Ocean Shield, is using a device from the United States that can be towed behind a ship and can pick up signals from deep underwater. A British ship, the HMS Echo, has similar capability.

The ships were moving slowly and searching to depths of almost 2 miles, said Commodore Peter Leavy, the commander of Australia’s Joint Task Force 658.

“The search using sub-surface equipment needs to be methodical and carefully executed in order to effectively detect the faint signal of the pinger,” he said.

Fourteen planes and 11 ships were involved in the search Friday, which covered a zone of about 83,000 square miles off Australia. Authorities reported some objects were spotted, but none was associated with Flight 370.

The plane disappeared four weeks ago on its flight from Kuala Lumpur, Malaysia, to Beijing.

— Erin McClam