It is increasingly common for terrorist groups not to claim responsibility for their actions, a leading expert says, amid heightened speculation one or both of the pilots may have been involved in diverting Malaysia Airlines flight MH370.
Greg Barton, the international director of the global terrorism research centre at Monash University, said there were several reasons a terrorist group might remain silent about hijacking the flight.
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''Perhaps this operation was only partially successful, and that the plan had been to turn back and crash into the Petronas Towers in Kuala Lumpur,'' Professor Barton said. ''Perhaps the pilots foiled the plan, we will never know.
''But that would be a motive for a group not to claim it, as they may want to try it again,'' he said.
Conjecture over pilot involvement in the plane's disappearance was fuelled on Sunday by a new timeline suggesting the flight's signalling system was disabled before a pilot spoke to air traffic control without mentioning any trouble.
But whether it was an act of terrorism remains a question that may not be answered unless the black box flight recorders are found.
Professor Barton cited the 1988 Lockerbie disaster, in which Pan Am flight 103 was destroyed by a bomb over a Scottish town, killing 270 people, as an example of an attack no one admitted ordering.
''It also took quite a while for al-Qaeda to claim responsibility for 9/11,'' Professor Barton said. ''And in the November 2008 attacks at several Mumbai hotels, Lashkar- e- Taiba was blamed but never actually claimed it,'' he said.
Clive Williams, a visiting professor at the Australian National University's centre for military and security law and an adjunct professor at Macquarie University's centre for policing, intelligence and counter terrorism said while terrorism could not be ruled out, it seemed less likely than other possibilities.
''Terrorism is by definition politically motivated with a strategic outcome in mind. If terrorism was the motivation you would expect that the perpetrators would have already used the plane as a weapon against a possible target, such as Mumbai or Colombo, would have made political demands, or would have tried to put pressure on a target government.''
Since 2000 there have been only 18 hijacks or attempted hijacks of large passenger aircraft. Of these, seven were by passengers wanting to get to a destination to seek asylum, one was criminally motivated to steal the cargo, six were by mentally ill persons, and four were politically motivated (counting September 11 as one incident), Professor Williams said.