Malaysian police are investigating whether hijacking, sabotage, or the crew and passengers' personal or psychological problems could be to blame for flight MH370's disappearance, they said on Tuesday.

"Other than mechanical problems, these are the main areas of concern," said Malaysia's inspector general, Tan Sri Khalid Abu Bakar.

Malaysian authorities have said they have not ruled out any explanations for the Malaysia Airlines plane going missing.

The police chief also said that one of the passengers travelling on a stolen passport had been identified as a 19-year-old Iranian man, believed to be travelling to Europe to seek asylum, and was not thought likely to be a member of a terrorist group. The other man has yet to be identified.Asked what he might mean by personal problems, he gave the example of someone who had bought a large insurance policy that would benefit family members.

Malaysia Airlines said authorities were searching the Malaysian peninsula, while the Vietnamese military said its units were hunting for any sign that the aircraft might have crashed into remote mountains or uninhabited jungle areas.

In a statement released on Tuesday, Malaysia Airlines said: "The authorities are looking at a possibility of an attempt made by MH370 to turn back to Subang. All angles are being looked at. We are not ruling out any possibilities."That was presumably a reference to Malaysia's Sultan Abdul Aziz Shah airport, also known as Subang airport. It lies not far to the north-west of Kuala Lumpur international airport, where the plane took off.

Malaysia's air force chief had previously said military radar suggested the aircraft might have made a U-turn. Lieutenant General Vo Van Tuan, deputy chief of staff of the Vietnamese army, told the Associated Press that authorities on land had been ordered to search for the plane, with military units also searching near the border with Laos and Cambodia.

"So far we have found no signs … so we must widen our search on land," he said.

Taiwan and New Zealand have joined the multinational hunt. The search area in the Gulf of Thailand and South China Sea – where the plane was first noticed missing from radar screens in the early hours of Saturday – has been expanded. Crews are also searching off the western coast of Malaysia and up towards the Andaman Sea because of the possibility that the plane turned back.

There have been fresh sightings of potential debris in the area but previous reports of oil slicks and possible wreckage have turned out to have no connection to the flight.

"Until now, with all of our efforts, there is very little hope for any good news about this plane," said Pham Quy Tieu, the head of Vietnam's search and rescue effort.

China had deployed 10 satellites using high-resolution earth imaging capabilities, visible light imaging and other technologies to assist the search, the People's Liberation Army Daily said.

The head of the organisation that monitors the nuclear test ban treaty said it was using its technology to check for any signs of an explosion close to where the flight went missing. Lassina Zerbo, who leads the Comprehensive Test Ban Treaty Organisation, said it was possible its infrasound sensors – which capture very low frequency acoustic waves – might have been able to detect a blast.

The United States has reviewed imagery from spy satellites for any glimpse of a mid-air explosion, without success.

Malaysia Airlines confirmed that maintenance 12 days before the Boeing 777 vanished had shown no signs of any problems. It said the plane was 12 years old and had flown for almost 53,500 hours.

Thai police played down the prospect that two men travelling on stolen passports were linked to the disappearance of the plane. "We haven't ruled it out but the weight of evidence we're getting swings against the idea that these men are or were involved in terrorism," Supachai Puikaewcome, chief of police in the Thai resort city of Pattaya, told Reuters.

The Bangkok Post said an employee at the travel agency said the man who booked the seats – an Iranian man known only as Mr Ali, who had often done business with the company – had originally asked for Etihad or Qatar Airways flights to Copenhagen and Frankfurt but then decided to book flights on China Southern because it was much cheaper.

Around two-thirds of the 227 passengers were Chinese and there were 12 Malaysian crew.

Agence France-Presse reported that some of the family members waiting for news at a hotel in Beijing were still clinging to hope despite being warned to prepare for the worst. "I hope it is a hijacking, then there will be some hope that my young cousin has survived," said a man in his 20s surnamed Su.

He added: "My uncle and aunt had an emotional breakdown – they are not eating, drinking and sleeping and could not face coming here. They need our help. We have been telling them lies – stressing the few positives in all of this – to keep their spirits up."

A man called Liu, whose older brother was on the flight, added: "The wait has been absolute torture."

As anger grew among the waiting relatives, Malaysia Airlines added in its statement: "We regret and empathise with the families and we will do whatever we can to ensure that all basic needs, comfort, psychological support are delivered. We are as anxious as the families to know the status of their loved ones.

"To the families of the crew on-board MH370, we share your pain and anxiety. They are of the MAS [Malaysia Airline System] family and we are deeply affected by this unfortunate incident."


-Guardian UK