Eight masked gunmen forced their way through the security fence at Brussels international airport, drove on to the tarmac and snatched diamonds worth 50 million US dollars (£32.3 million) from the hold of a plane without firing a shot.
The gang responsible for one of the biggest diamond heists in recent years used two black vehicles with flashing blue police lights in the raid on Monday, said Anja Bijnens, spokeswoman for the Brussels prosecutors' office.
"They tried to pass themselves off as police officers," Ms Bijnens said.
The thieves, 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 mini-van believed to be involved in the robbery near the airport later on Monday night.
The heist was estimated at some 50 million US dollars in diamonds, said Caroline De Wolf of the Antwerp World Diamond Centre. She added: "What we are talking about is obviously a gigantic sum."
The robbers forced their way through a fence, at a place where two work sites obstructed a clear view, Ms 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 Ms Bijnens. The robbers finished their clinical operation with a high-speed departure through the same hole in the fence, completing the theft within barely five minutes, she added.
Airport spokesman Jan Van Der Cruijsse could not explain how the area could be so vulnerable to theft. He said: "We abide by the most stringent rules." 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 Centre, yielding loot that police in 2003 estimated to be worth about 100 million US dollars at the time.