BRUSSELS (AP) ― When the armored car set off for the Brussels airport carrying $50 million worth of precious stones from Antwerp’s diamond district, eight gunmen knew all about it.

One of the biggest diamond heists in recent memory was about to go down.

The thieves surely knew it would be too risky to make their move in Antwerp, which is the world capital of diamond-cutting, 43 kilometers from the airport. The city’s diamond industry has some 2,000 surveillance cameras, police monitoring and countless identity controls to protect its $200 million in daily trade of rough and polished gems.

“We are just about the safest place in Belgium,” said Antwerp World Diamond Center spokeswoman Caroline De Wolf.

And the thieves no doubt realized that once Swiss Flight LX789 was airborne Monday night on its way to Zurich, the diamonds tucked in its hold, it would be too late to get their hands on the gems.

But the airport’s 25-kilometer fence and the transfer of the diamonds from the armored car on the tarmac to the hold of the Fokker 100 twin-engine jet ― now that held potential.

After weeks of lashing rain, snow, sleet and black ice, Monday evening was finally as good as it gets in late winter in Belgium. Crisp, cold air meant dry roads for a perfect getaway, and winter’s early darkness was a blessing for those needing stealth.

About 20 minutes before the flight’s scheduled 8:05 p.m. departure, the robbers hid in a construction site outside the airport fence. Then they apparently cut through the fence and, in two black cars with blue police lights flashing, drove onto the tarmac, speeding straight to pier A, where the armored car had just finished transferring the diamonds to the Fokker.

Dressed in dark police clothing and hoods, the thieves whipped out machine guns and stopped the pilots and the transport security crew in their tracks.

“They saw nothing,’’ Anja Bijnens of the Brussels prosecutor’s office said Tuesday. The thieves “never fired a shot. They never injured anyone.”

With speed and precision, the thieves opened the plane’s hold, picked out 120 parcels and loaded them into the cars.

“Afterward, they made a high-speed getaway,’’ Bijnens said, estimating the whole operation took five minutes.

By late Tuesday, investigators had found the charred remains of a van most likely used in the heist but little else.