The prime ministers of Australia and Malaysia expressed concern for still-waiting families.

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KUALA LUMPUR, Malaysia (USA TODAY) - Police chief Khalid Abu Bakar said Thursday that the investigation into a missing flight is focusing on unnamed crew members and four possible scenarios: hijacking, sabotage, psychological problems and personal problems.

Khalid declined to elaborate on who they are focusing on, saying he did not want to hamper the investigation.

Meanwhile, Malaysian Prime Minister Najib Razak was in Australia on Thursday to meet with Prime Minister Tony Abbot, whose country has been leading search efforts for Flight 370 in the southern Indian Ocean.

Razak said declared the ongoing search for missing flight MH370 profoundly difficult and expressed concern for the families still waiting for news, during a joint press conference in Perth, Australia, Wednesday night.

Razak is visiting Perth, which is 932 miles east of the focus of the search for the flight that vanished March 8 en route from Kuala Lumpur to Beijing. There were 239 people on board.

The hunt for the Boeing 777 in a remote part of the Indian Ocean is the "most difficult search ever undertaken," Abbott said.

He also expressed sympathy for the families who are still waiting for evidence of what might have happened to the missing plane.

"I know this is an extraordinarily difficult and devastating time for you but we will not let you down," Abbott said. "We will spare no effort … we will not rest until we've done everything that we humanly can."

Razak said hundreds have been involved in the search as have dozens of countries.

"They have searched through stormy seas … they have sailed through storms to find the plane," Razak said. "This has been a remarkable effort bringing together nations from around the world."

Nine ships are searching the Indian Ocean for signs of the jet, Razak said.

"I know until we find the plane, many families cannot start to grieve," Razak said. "I cannot imagine what they are going through."

and stopped at the military base where search aircraft are based.

The jet vanished on March 8 on the way from Kuala Lumpur to Beijing carrying 239 people. Nothing from the Boeing 777 flight has been recovered.

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