* Hijack, sabotage, mechanical failure also being investigated
* Still no sign of missing plane, search in fourth day
* 227 passengers and 12 crew presumed dead aboard lost flight
* 19-year-old Iranian holding a stolen passport aboard the plane
Malaysia is investigating whether any passengers or crew aboard a missing airliner had personal or psychological problems that might explain its disappearance, along with the possibility of a hijack, sabotage or mechanical failure, police said on Tuesday.
A massive search operation for the Malaysia Airlines Boeing 777-200ER is in its fourth day, with no trace yet of the aircraft or the 239 people on board, one of the most baffling mysteries in aviation history.
Flight MH370 left Kuala Lumpur for Beijing early on Saturday morning, vanishing from radar screens about an hour after take-off over the sea separating Malaysia from the southern tip of Vietnam. Adding to the puzzle, Malaysian military radar tracking suggested it may have turned back from its scheduled route.
There was no distress signal or radio contact indicating a problem and, in the absence of any wreckage or flight data, police have been left trawling through passenger and crew lists for potential leads.
"Maybe somebody on the flight has bought a huge sum of insurance, who wants family to gain from it or somebody who has owed somebody so much money, you know, we are looking at all possibilities," Malaysian police chief Khalid Abu Bakar told a news conference.
"We are looking very closely at the video footage taken at the KLIA (Kuala Lumpur International Airport), we are studying the behavioral pattern of all the passengers."
The fact that at least two passengers on board had used stolen passports, confirmed by Interpol, has raised suspicions of foul play. But Southeast Asia is known as a hub for false documents that are also used by smugglers, illegal migrants and asylum seekers.
Police chief Khalid said one of the men had been identified as a 19-year-old Iranian, who appeared to be an illegal immigrant. The identity of the other was still being checked.
"We believe he is not likely to be a member of any terrorist group, and we believe he was trying to migrate to Germany," Khalid said of the teenager. His mother was waiting for him in Frankfurt and had been in contact with authorities, he said.
Asked if that meant he ruled out a hijack, Khalid said: "(We are giving) same weightage to all (possibilities) until we complete our investigations."
Both men entered Malaysia on Feb 28, at least one from Phuket, in Thailand, eight days before boarding the flight to Beijing, Malaysian immigration chief Aloyah Mamat told the news conference. Both held onward reservations to Western Europe.
Police in Thailand, where the passports were stolen and the tickets used by the two men were booked, said they did not think they 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.
OT A TRACE
The massive search for the plane has drawn in navies, military aircraft, coastguard and civilian vessels from 10 nations.
The search was widened on Tuesday to cover a larger swathe of the shallow waters of the Gulf of Thailand and South China Sea around the last known position of the plane.
But searches were also being conducted on the western coast of Malaysia and northwest towards the much deeper Andaman Sea - based on a theory that the plane may have flown on for some time after deviating from its flight path.
"This will be a long search. We need a long-term search plan," Do Ba Ty, Vietnam's army chief of staff and deputy defence minister, told reporters.
"We will expand search to the east in the sea and to the west on land and ask for Cambodia help...we will go where our friends go and make sure we inform our citizens and fishermen to request their help in the search."
Neither Malaysia's Special Branch, the agency leading the investigation, nor spy agencies in the United States and Europe, have ruled out the possibility of a hijack or bombing.
But Malaysian authorities have indicated the evidence so far does not strongly back an attack as a cause for the aircraft's disappearance, and that mechanical or pilot problems could have led to the apparent crash, U.S. government sources said.
"There is no evidence to suggest an act of terror," said a European security source, who added that there was also "no explanation what's happened to it or where it is".
The United States extensively reviewed imagery taken by spy satellites for evidence of a mid-air explosion, but saw none, a U.S. government source said.
Vietnam said it was allowing ships and planes from Malaysia, Singapore, China and the United States to enter its waters to search for the plane.
MOST PASSENGERS CHINESE
About two-thirds of the 227 passengers and 12 crew now presumed to have died aboard the plane were Chinese. Other nationalities included 38 Malaysians, seven Indonesians, six Australians, five Indians, four French and three Americans.
China has deployed 10 satellites using high-resolution earth imaging capabilities, visible light imaging and other technologies to "support and assist in the search and rescue operations", the People's Liberation Army Daily said on Tuesday.
The Boeing 777 has one of the best safety records of any commercial aircraft in service. Its only previous fatal crash came on July 6 last year when Asiana Airlines Flight 214 struck a seawall on landing in San Francisco, killing three people.
U.S. planemaker Boeing has declined to comment beyond a brief statement saying it was monitoring the situation.