The gunmen held the pilot, co-pilot and several others at gunpoint during the robbery Monday at the international airport in Brussels, although no shots were fired and no one was injured, VRT Network reported.
Most of the loot removed from the Swiss airline plane was diamonds, valued by appraisers at about $50 million, police and prosecutors said.
Officials said the plane was headed to a destination in Switzerland.
The robbery prompted the police union to question the security provided by three law enforcement agencies at the airport, VRT said.
The Antwerp diamond sector also expressed concern about the airport robbery.
"It is quite remarkable that such a thing could happen," said Caroline Wolf of the Antwerp Diamond Trade Center. "It is a problem."