European officials said it appeared two people on board were using stolen passports and Malaysian Transport Minister Hishamuddin Hussein said authorities were also checking the identities of two other passengers.
"All the four names are with me," said Hishamuddin, who is also defence minister. "I have indicated to our intelligence agencies and I have also spoken to international intelligence agencies for assistance."
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.
- Two on missing Malaysia jet used stolen EU passports
- Malaysia Airlines plane missing, presumed crashed
Malaysia's air force chief says that military radar indicated the missing Boeing 777 jet may have turned back, but declined to give further details on how far the plane may have veered off course.
Rodzali Daud told a press conference Sunday that "there is a possible indication that the aircraft made a turnback," adding that authorities were "trying to make sense of that.
Malaysia Airlines Chief Executive Ahmad Jauhari Yahya said the pilot is supposed to inform the airline and traffic control authorities if he does return, but that officials had received no such distress call.
Search effortsInternational actors have cooperating expediently in the search for the missing jet. 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 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."
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.
Reuters contributed to this report