BRUSSELS—Eight masked gunmen forced their way through the security fence at Brussels' international airport, drove onto the tarmac and snatched some $50 million worth of diamonds from the hold of a Swiss-bound plane without firing a shot.

The gang responsible for one of the biggest diamond heists in recent years used two black vehicles with a flashing blue police lights in their daring raid late Monday, said Anja Bijnens, spokeswoman for the Brussels prosecutor's office.

“They tried to pass themselves off as police officers,” Bijnens said Tuesday. The robbers, who wore outfits resembling dark police clothing, got away with 120 parcels, mostly containing diamonds but some also holding precious metals.

Police said they found a burnt-out minivan believed to be involved in the robbery near the airport later Monday night.

The heist was estimated at some $50 million in diamonds, said Caroline De Wolf of the Antwerp World Diamond Center. “What we are talking about is obviously a gigantic sum,” De Wolf said.

The robbers forced their way through a perimeter fence, at a place where two work sites obstructed a clear view, Bijnens said. There were no details about how the hole was opened but airport authorities said it must have taken more than simply blasting through it with a vehicle.

The robbers drove up to the Swiss passenger plane some 20 minutes before departure time, brandishing their machine-guns. Then they methodically broke into the hold, which was accessed from outside, to choose their loot.

Passengers were unable to see the drama beneath them, said Bijnens.

The robbers finished their clinical operation with a high-speed departure through the same hole in the fence, completing the spectacular theft within barely five minutes, Bijnens said.

Airport spokesman Jan Van Der Cruijsse could not explain how the area could be so vulnerable to theft. “We abide by the most stringent rules,” he said.

The Swiss flight, bound for Zurich and operated by Helvetic Airways, was cancelled.

A decade ago the port 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 at the time.

Monday's heist though was a fresh blow to the Antwerp industrial diamond centre which prides itself on security and discretion.

“This is causing quite some unrest,” said De Wolf. “It was incredible how easy it all went. This is worrying in terms of competitiveness, since other diamond centres are ready to pounce and take over our position.”

Antwerp's Diamond Center stands in the heart of the high-surveillance diamond district where police and hundreds of cameras work around the clock, and security has been beefed up further since the spectacular 2003 robbery. Shipments to the airport leave aboard armoured trucks on an almost daily basis.

The shipment was not extraordinary, since on any given day, some $200 million in polished and rough stones go through the Antwerp diamond centre.

Monday's parcels contained rough and polished stones heading for Switzerland, where many of the 120 parcels were intended for different handlers.

The insurance for air transport — handled sometimes by airlines themselves or external insurance companies — is usually relatively cheap because it's considered to be the safest way of transporting small high value items, logistics experts say.

Unlike a car or a truck, an airplane cannot be waylaid by robbers once it's on its way, and it is considered to be very secure before the departure and after the plane's arrival because the aircraft is always within the confines of an airport — which are normally 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.

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“It does seem very worrying that someone can actually have the time to drive two vehicles onto the airport, effect the robbery, and drive out without being intercepted,” Baum said.

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Juergen Baetz and Don Melvin contributed to this report from Brussels.