By Julie Makinen, Los Angeles Times

BEIJING — Private companies specializing in deep ocean search will be hired to continue the quest to find Malaysia Airlines Flight 370, Australia’s prime minister said Monday, and the cost could run more than $55 million and take six to eight months.

The U.S. Navy’s robotic submarine Bluefin-21 has searched more than 150 square miles of the floor of the Indian Ocean in an area where investigators thought they had detected pings from the Boeing 777’s black box transmitters in early April. We are “baffled and disappointed” that no wreckage had been found 52 days into the search, Prime Minister Tony Abbott said at a news conference in Canberra, Australia.

Now, Abbott said, the undersea search area will be expanded from the most probable impact zone to a much wider area totaling 21,621 square miles.

Side-scan sonar devices towed behind ships will be used to traverse the expanded search area, said retired Australian Air Chief Marshal Angus Houston, who has been coordinating search efforts from Perth, Australia.

Abbott indicated that air searches for surface debris would be discontinued imminently. “It is highly unlikely we will find any debris on the ocean surface” now, he said, explaining that any material from the plane probably would have become waterlogged and sunk by this time.

“We are moving to a phase focused on searching the ocean floor over a much larger area,” Abbott said. So far, the visual search area has encompassed 334 flights totaling more than 3,000 hours and scoured 1.7 million square miles of the ocean surface.

Until the private companies can begin working, Abbott said, the Bluefin-21 as well as Australian, Malaysian and Chinese ships will continue operating in the search area. An Australian military plane will remain on standby in case suspected wreckage is identified, he added.

“I want the families to know, I want the world to know, Australia will not shirk its responsibility,” Abbott said. “We will not let people down.”

Asked about his comments several weeks ago that investigators were close to finding the wreckage, Abbott said he was “not in the business of making excuses for failure” but added that the search was perhaps the most difficult in history.

“Enormous efforts have been made,” he said, describing the jet’s disappearance as an “extraordinary mystery.”

Houston said investigators believe they are looking in the right area. “We were quietly optimistic,” he said, that the Bluefin-21 would find the jet wreckage.

But he noted that it took more than two years to find the undersea wreckage of Air France Flight 447 in the Atlantic Ocean after it crashed in 2009, and that in that case, the plane was found just 6.5 nautical miles from its last known position. Data about the last known position of Malaysia Airlines Flight 370 is much less robust.

Asked about the cost of the search so far, Abbott declined to provide specifics, saying authorities had been using military assets that governments would be “paying for anyway.”

The Boeing 777 vanished March 8 en route from Kuala Lumpur, Malaysia, to Beijing with 239 people aboard. Authorities have said they believe a deliberate act by someone aboard the aircraft led to its disappearance, but they have not offered more specifics.

AFP/Mohd Rasfan