Malaysian Airlines Flight MH370 may have been hijacked by terrorists who stowed away in its underfloor electronics bay located just behind the cockpit.
According to trade journal Aviation Security editor Philip Baum, the hijackers may not have been on a passenger list but hiding aboard the Boeing 777.
"Stowaways on planes are not new," he said. "Whilst most stowaways had clambered into wheel wells, some had secreted themselves onboard aircraft, often disguised as airport staff."
On July 7, 2012, two stowaways penetrated Iceland's Keflavik International Airport dressed as airport staff and boarded an Icelandair aircraft.
But Mr Baum said "they were found by alert crew members prior to departure".
"The same cannot be said in the case of a China Airlines flight which landed in San Francisco on October 22, 2012," he said.
He said a passenger had flown from Shanghai to Taipei and on to the US where he claimed asylum.
The passenger wore a cleaner's uniform to board the plane and avoided detection in Taipei by hiding in an electrical compartment.
Mr Baum said the industry had long been concerned about the insider threat. "At almost every major international airport in the world, criminal activity of one type or another takes place in what are supposed to be sterile zones," he said.
"It is certainly a possibility that, in an airport the size of Kuala Lumpur, individuals, with or without the knowledge of the crew of MH 370, and with or without technical knowledge as to how to disable an aircraft's communication systems, could have managed to secrete themselves onboard."