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UALA LUMPUR, Malaysia (AP) -- Planes and ships from across Asia resumed the hunt Sunday for a Malaysian jetliner missing with 239 people on board for more than 24 hours, while Malaysian aviation authorities investigated how two passengers were apparently able to get on the aircraft using stolen passports.
There was still no confirmed sighting of wreckage from the Boeing 777 in the seas between Malaysia and Vietnam where it vanished from screens early Saturday morning en route to Beijing from Kuala Lumpur. The weather was fine, the plane was already cruising and the pilots had no time to send a distress signal - unusual circumstance for a modern jetliner to crash.
Li Jiaxiang, administrator of the Civil Aviation Administration of China, said some debris had been spotted, but it was unclear whether it came from the plane. Vietnamese authorities said they had seen nothing close to two large oil slicks they saw Saturday and said might be from the missing plane.
Malaysia's civil aviation chief Azaharuddin Abdul Rahman said his country had expanded its area of operation to the west coast of peninsular Malaysia, on the other side of the country from where the plane disappeared. "This is standard procedure. If we can't find it here, we go to other places," he said.
Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over many square kilometers (miles). If the plane enters the water before breaking up, there can be relatively little debris.
Investigators will need access to the flight data recorders to determine what happened.
Terrorism is always considered a possibility, but the sudden disappearance of Flight MH370 has given extra emphasis to speculation a bomb might have been on board. Other scenarios include some catastrophic failure of the engines or structure of the plane, extreme turbulence or even pilot suicide.
On Saturday, foreign ministries in Italy and Austria said the names of two citizens listed on the flight's manifest matched the names on two passports reported stolen in Thailand. It's unclear how common it is for people to get on flights with fake passports, but the news added to fears of terrorism.
Azaharuddin said Sunday that authorities were "aware of the situation and we are doing an investigation at the moment."
Malaysia Airlines has a good safety record, as does the 777, which had not had a fatal crash in its 19-year history until an Asiana Airlines plane crashed last July in San Francisco, killing three passengers, all teenagers from China.
Prof. Jason Middleton, the head of the Sydney-based University of New South Wales' School of Aviation, said terrorism or some other form of foul play seemed a likely explanation.
"You're looking at some highly unexpected thing, and the only ones people can think of are basically foul play, being either a bomb or some immediate incapacitating of the pilots by someone doing the wrong thing and that might lead to an airplane going straight into the ocean," Middleton said on Sunday. "With two stolen passports (on board), you'd have to suspect that that's one of the likely options."
Just 9 percent of fatal accidents happen when a plane is at cruising altitude, according to a statistical summary of commercial jet accidents done by Boeing. Malaysia Airlines CEO Ahmad Jauhari Yahya said Saturday there was no indication the pilots had sent a distress signal.
The plane was last inspected 10 days ago and found to be "in proper condition," Ignatius Ong, CEO of Malaysia Airlines subsidiary Firefly airlines, said at a news conference.
Two-thirds of the jet's passengers were from China. The rest were from elsewhere in Asia, North America and Europe.
Asked whether terrorism was suspected, Malaysian Prime Minister Najib Razak said authorities were "looking at all possibilities, but it is too early to make any conclusive remarks."
Greg Barton, a professor of international politics at Australia's Monash University and a terrorism expert, said if the disaster was the result of terrorism, there is no obvious suspect. If it was terrorism, Barton expected China would be quick to blame separatists from the ethnic Uighur minority, as authorities did recently when 29 people were killed in knife attacks at a train station in the southern city of Kunming.
"If a group like that is behind it, then suddenly they've got a capacity that we didn't know they had before, they've executed it very well - that's very scary," Barton told AP. "It's safe to start with the assumption that that's not very likely, but possible."
Brummitt reported from Hanoi, Vietnam.
Associated Press journalist Rod McGuirk in Canberra; Didi Tang and Aritz Parra in Beijing; Stephen Wright in Bangkok; Colleen Barry in Milan; George Jahn in Vienna; Jim Gomez and Oliver Teves in Manila, Philippines; Joan Lowy in Washington; and Scott Mayerowitz in New York also contributed this report.