Unlike a car or a truck, an airplane cannot be attacked by robbers once it's on its way, and it is considered to be very safe before the departure and after the plane's arrival because the aircraft is always within the confines of an airport - a place usually highly secured.

Philip Baum, an aviation security consultant in Britain, said the robbery was worrying not because the fence was breached, but because the response did not appear to have been immediate. That, he said, raised questions as to whether alarms were ringing in the right places.

That amount of time would also allow someone to board the plane, he said.

A decade ago, the Belgian city of Antwerp, the world capital of diamond-cutting, was the scene of what was probably one of the biggest diamond heists in history, when robbers took precious stones, jewels, gold and securities from the high-security vaults at Antwerp's Diamond Center, yielding loot that police in 2003 estimated to be worth about $100 million.

Antwerp's Diamond Center stands in the heart of the high-surveillance diamond district where police and dozens of cameras work round-the-clock, and security has been beefed up further since the spectacular 2003 robbery.