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PERTH, Australia -- The Malaysian government said late Monday that the final words received by ground controllers from the missing Malaysia Airlines Boeing 777 on March 8 were "Good night Malaysian three-seven-zero," not the previously stated and slightly unusual, "all right, good night."

The statement didn't explain the discrepancy. The statement also said investigators were still trying to determine whether it was the pilot or co-pilot who spoke the words at 1:19 a.m., just before the plane went out of contact and disappeared from radar.

STORY:Malaysia Airlines search 'could drag on'

Malaysia has been criticized for its handling of the search, particularly its communications to the media and families of the passengers. The sudden and unexplained change in the account of the last transmission from the cockpit was likely to fuel those concerns.

The three-week hunt for Flight 370 has turned up no sign of the Boeing 777, which vanished with 239 people on board bound for Beijing from Kuala Lumpur.

As the race to find debris from the plane before the blackbox stops sending out a "ping" signal ramped up this week, Australia said Tuesday that it would deploy a modified Boeing 737 to act as a flying air traffic control center over the Indian Ocean, to prevent a mid-air collision among the aircraft searching for the missing jetliner.

An air force E-7A Wedgetail equipped with advanced radar was to be deployed "in the near future" to monitor the increasingly crowded skies over the remote search zone, said Angus Houston, who heads the joint agency coordinating the multinational search effort.

At a news conference in Perth, Houston, the former Australian defense chief, called the search effort the most challenging one he has ever seen. The starting point for any search is the last known position of the vehicle or aircraft, he said.

"In this particular case, the last known position was a long, long way from where the aircraft appears to have gone," he said. "It's very complex, it's very demanding."

"What we really need now is to find debris, wreckage from the aircraft," he said. "This could drag on for a long time."

The search zone area has shifted as experts analyzed Flight 370's limited radar and satellite data, moving from the seas off Vietnam to the waters west of Malaysia and Indonesia, and then to several areas west of Australia. The current search zone is a remote 98,000 square mile that is a roughly 2 1/2 hour flight from Perth.

Under normal circumstances, ground-based air traffic controllers use radar and other equipment to keep track of all aircraft in their area of reach, and act as traffic policemen to keep planes at different altitudes and distances from each other. This enforced separation - vertical and horizontal - prevents mid-air collision. But the planes searching for Flight 370 are operating over a remote patch of ocean that is hundreds of miles from any air traffic controller.

On Tuesday, 11 planes and nine ships were focusing on less than half of the search zone, some 46,000 square miles of ocean west of Perth, with poor weather and low visibility forecast, according to the new Joint Agency Coordination Center, which will oversee communication with international agencies involved in the search. A map from the center showed that the search area was about 1,200 miles west of Perth.

The arrival of the E-7A "will assist us with de-conflicting the airspace in the search area," Houston told reporters. He did not specify when the mid-air plane would be deployed. The plane can maintain surveillance over a surface area of 156,000 square miles at any given time, according to the air force's website.

Rob Shearer, captain of the Royal New Zealand Air Force's P-3 Orion, on Monday warned his crew to stay alert for the growing number of planes and ships crisscrossing the area. Some of the search aircraft have been dropping as low as 200 feet above the water -- and occasionally dipping even lower for brief periods.

"An important note to mention to all of you, there's a lot of surface craft out there now, so we need to know and have eyes on everything before we go below 1,000 (feet)" Shearer told his crew before they headed out to the search zone.

Although it has been slow, difficult and frustrating so far, the search for the missing Malaysia Airlines jet is nowhere near the point of being scaled back, Australian Prime Minister Tony Abbott said Monday. In fact, he said the intensity and magnitude of operations "is increasing, not decreasing."

"I'm certainly not putting a time limit on it," Abbott said from at RAAF Pearce, the Perth military base coordinating the operation. "We can keep searching for quite some time to come."

"We owe it to the families, we owe it to everyone that travels by air. We owe it to the anxious governments of the countries who had people on that aircraft. We owe it to the wider world which has been transfixed by this mystery for three weeks now," he said.

"If this mystery is solvable, we will solve it," Abbott said.

Items recovered so far were discovered to be flotsam unrelated to the Malaysian plane. Several orange-colored objects spotted by plane Sunday turned out to be fishing equipment.

© 2014 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.

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