Is the case of the missing Malaysia Airlines plane a bid to stage an attack similar to the 9/11 attacks, with perpetrators hoping to crash the jet into buildings in a major city in the region?
Is the plane's disappearance engineered by a lone-wolf operator or a group with terrorist links?
These are some of the questions being discussed in terrorism analysis circles as a probe deepens into the case of Flight MH370, which went missing in the early hours of March 8 with 239 people on board, many of them from China.
In particular, the theory of a 9/11-style attack was triggered by revelations last week from Al-Qaeda informant Saajid Badat that a group of Malaysians had been planning to take control of a Malaysia Airlines plane by blowing up the cockpit door with a bomb hidden in a shoe.
He said he had given the Malaysians a shoe bomb to "access the cockpit" in the 2002 plot, according to British media.
He was speaking to a court in the United States via video link in the trial of Osama bin Laden's son-in-law, in the 9/11 case.
Some aviation experts and pilots suggested that the MH370 cockpit could have been compromised and pilots could have deliberately crashed the plane to prevent a 9/11-style attack.
Meanwhile, former US Deputy Secretary of State Strobe Talbott added to concerns at the weekend with his tweet that MH370 may have been hijacked for a 9/11-type attack in India.
Terrorism analysts said the possibility of a terror attack would have to be ruled out by investigators, but added that evidence so far does not clearly suggest it was one.
Malaysian Prime Minister Najib Razak had said at the weekend that "deliberate" action was likely taken to disable the plane's communication systems and to turn the airliner back across Peninsular Malaysia, from its position to the east.
But the absence of interceptions and Web chatter, and the limited capability of Muslim militant groups such as the Al-Qaeda-linked Jemaah Islamiah (JI) of Indonesia, or the Uighur separatists of China's Xinjiang province, are leading analysts to be circumspect, pending new evidence that points to a terrorist attack.
"An incident of this magnitude has to have a trail," said Singapore-based terrorism analyst Rohan Gunaratna.
"If pilot error and mechanical failure have been discounted, and the Malaysian Prime Minister has said it was deliberate, the possibilities are that it could be a criminal act, a terrorist attack or one driven by psychological stress," said the head of the International Centre for Political Violence and Terrorism Research.
"If it was criminal, where is the financial trail? If terrorism was the motive, why is there no intelligence yet?
"Background checks on all crew members and passengers will have to be conducted to rule out psychological motives."
A shoe bombing attempt is unlikely as the shoe bomb mentioned by Badat was found, and the plot was in the works several years ago, he added.
Established groups such as JI have lost their capability to plan large-scale operations after the systematic efforts of governments in South-east Asia to destroy their infrastructure, said London-based analyst Sajjan Gohel.
Those associated with the group are confined to operations on Indonesia's outlying islands.
"We know that the Uighurs attacked civilians at a railway station, but they are not known to have planned an operation of this scale," he said, referring to the missing Malaysia Airlines plane.
"It was a very sophisticated operation. That leaves open the possibility of a lone-wolf operator.
"And we have seen a few of them recently, in the Boston marathon attacks and earlier in Norway."
The bomb attack in Boston 11 months ago by two brothers left three people dead and 364 injured, while Norwegian political extremist Anders Breivik's lone actions in 2011 in Norway left 77 dead.
Dr Gohel believes the pilots of the missing Malaysian jet most likely headed towards the Indian Ocean because the route would mean a lower possibility of being detected by military radars.
But if that is true, it is possible that the world would not get to know what exactly happened.
"The waters there are the deepest in the world, with deadly sharks. It would be unsafe for anyone to try to reach the debris. It could be hazardous," he said.
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