There are continuing fears that the missing Malaysian flight may have been the victim of a terrorist attack as the FBI joins Interpol's search for the Beijing-bound plane.
Flight MH370 has been missing since it went off the radar over the South China Sea in the early hours of Saturday (8 March) with 239 passengers and crew members on board.
The Boeing 777 was cruising at more than 35,000ft when it disappeared in reportedly good weather. No distress signal was made by the pilots and no debris from the plane has yet been found.
Reports of a "yellow object", thought to be a life raft from the missing Malaysia Airlines plane have been dismissed by the Vietnam Civil Aviation Body.
The FBI, Interpol, Malaysia and Beijing are investigating whether the flight was blown up or crashed following a botched hijacking, reports The Times.
Two of the passengers who boarded the flight from Kuala Lumpur to Beijing did so with stolen passports, resulting in Interpol launching an attack on Malaysian Airlines for lax security checks.
"We have a real case where the world is speculating whether the stolen passport holders were terrorists, while Interpol is asking why only a handful of countries worldwide are taking care to make sure that persons possessing stolen passports are not boarding international flights," said Interpol's security general Ronald Noble.
He added: "If Malaysia Airlines and all airlines worldwide were able to check the passport details of prospective passengers against Interpol's database, then we would not have to speculate whether stolen passports were used by terrorists to board MH 370. We would know that stolen passports were not used by any of the passengers to board that flight."
The passengers using the stolen passports were said to be of Asian appearance, but using the names on an Italian and on an Austrian passport — Luigi Maraldi and Christian Kozel respectively.
Despite no group coming forward to claim responsibility and fraudulent passports more commonly used within regional flights, terrorism experts described the news surrounding the two fake passports as a "serious issue".
Dr Sally Livesley, a specialist in catastrophic and extreme risk and managing director of Newrisk Limited, told ITV's Daybreak: "The fact that there were two people is a serious, about whether it was a suicide bombing attack.
"What we find now is that people can still take a small amount of liquids, they can take any amount of powder, and their mobile phones, all of which can help with a bomb on board."
Azharuddin Abdul Rahman, Malaysia's civil aviation chief, also said officials have not ruled out a terrorist attack or botched hijacking explaining the plane's disappearance.
Scott Hamilton, managing director of aviation consultancy Leeham Co, added: "Either you had a catastrophic event that tore the airplane apart, or you had a criminal act. It was so quick and they didn't radio."
US officials have downplayed any suggestions the flight was hit by a terrorist attack.
- Missing Malaysia Airlines Jet: Five People Checked in But Did Not Board MH370
- Missing Malaysia Airlines Flight MH370: Passengers' Mobile Phones Ring But Not Answered
- Malaysia Airlines Flight May Have Disintegrated Mid-Air say Senior Investigators
- Malaysia Airlines Flight MH370: The Search for Missing Plane in Pictures
"Just because they (the passports) were stolen doesn't mean the travellers were terrorists," a Department of Homeland Security official told the Los Angeles Times.
"They could have been nothing more than thieves. Or they could have simply bought the passports on the black market."
A spokesperson for Malaysian Airlines said: "The airline continues to work with the authorities and we appreciate the help we are receiving from all parties and agencies during this critical and difficult time especially the members of the media.
"Malaysia Airlines reiterates that it will continue to be transparent in communicating with the general public via the media in all matters affecting MH370.
"Malaysia Airlines is similarly anxious and we appreciate the patience, support and prayers from everyone."