BRUSSELS — They arrived at Brussels Airport armed with automatic weapons and dressed in police uniforms aboard two vehicles equipped with blue police lights. But their most important weapon was information: the eight hooded gangsters who on Monday evening seized diamonds worth tens of millions of dollars from a passenger plane preparing to depart for Switzerland knew exactly when to strike — just 18 minutes before takeoff.

Forcing their way through the airport’s perimeter fence, the thieves raced, police lights flashing, to Flight LX789, which had just been loaded with diamonds from a Brink’s armored van from Antwerp, Belgium, and was getting ready for an 8:05 p.m. departure for Zurich.

“There is a gap of only a few minutes” between the loading of valuable cargo and the moment the plane starts to move, said Caroline De Wolf, a spokeswoman for the Antwerp World Diamond Centre, an industry body that promotes the diamond business in Belgium. “The people who did this knew there was going to be this gap and when.”

They also knew they had to move swiftly in a secure airport zone swarming with police officers and security guards. Waving guns that the Brussels prosecutors’ office described as “like Kalashnikovs,” they calmly ordered ground staff workers and the pilot, who was outside the plane making a final inspection, to back off and began unloading scores of gem-filled packets from the cargo hold. Without firing a shot, they then sped away into the night with a booty that the Antwerp Diamond Centre said was worth around $50 million but which some Belgian news media reported as worth much more.