The two thieves snuck into the rhino gallery and ripped a stuffed head off the wall. They carried it to a restroom, opened a window, and dropped the 30-kilo (66-pound) trophy two stories down to an accomplice waiting in a van.
"For 80 years we took care of it and from one day to the next it's no longer there," said Georges Lenglet, vertebrate exhibit curator at the Brussels museum, who has little hope of seeing the head again.
The museum had never been robbed until the July heist, when it became the latest of a rising number of science museums in Europe targeted by thieves for rhino horns, which can fetch tens of thousands of euros on the black market.
"It's a nasty little piece of criminal activity," Patrick Byrne, head of the organised crime networks unit at the European police agency Europol, told AFP.
Europol suspects an Irish organised crime group is behind a spate of robberies that has hit not only museums but also zoos, auction houses, antique dealers and private collectors across the continent over the past 18 months.
The gang, known to use violence and intimidation, is involved in drug trafficking, money laundering and smuggling of counterfeit products, but has seized too on a lucrative niche market in the sale of rhino horns.
Scotland Yard says the spike in museum thefts is driven by a significant increase in the value of rhino horns in Asia. Depending on its size, a horn can sell for 25,000 to 200,000 euros (34,000 to 288,000 dollars), according to Europol.
The horns are usually ground into powder and end up in the Asian market where they are prized for purported medicinal virtues to cure fevers, headaches, typhoid and smallpox. Their use for impotence is merely a myth.
The emergence of museum horn thefts coincides with an alarming surge in poaching of live rhinos in Africa.
More than 200 rhinos have been killed so far in 2011, after 333 were slaughtered in 2010, up from 122 in 2009, 83 in 2008, 13 in 2007, according to Save the Rhino International, a London-based conservation group.
Lucy Boddam-Whetham, Save the Rhino's acting director, fears the robberies will only exacerbate the illegal trade.
"It's stimulating more demand and stimulating the market, not taking pressure away from live rhinos," she told AFP.
Robberies have been reported by museums in Portugal, France, Germany, Britain, the Czech Republic and as far north as Sweden. According to Scotland Yard, 20 thefts have taken place across Europe in just the past six months.
They have prompted curators to beef up security systems or even remove rhinos from display.
The Brussels Royal Institute for Natural Sciences Museum did both after the black rhino head, which dated from 1827, was stolen just three weeks after a similar heist failed in the Belgian southern city of Liege.
"It's quite sad," said white-haired, bespectacled Lenglet in front of a display window now featuring two whole rhinos instead of three, and one head rather than two, after the museum locked away its most precious specimens.
The gang had clearly done its homework.
While one man distracted the guard by asking for information, two others picked the lock to the display door. By using the restroom window, the gang found a quick way to get the piece out without going through the front door.
The suspect Irish gang has used both crude "smash and grab" techniques and violence to snatch rhino heads, or more sophisticated burglaries based on meticulous surveillance and reconnaissance work, Europol's Byrne said.
"These people are indiscriminate in their criminal methods," he said. The rhino heads are quickly sold on the black market, and the gang launders the cash by purchasing real estate or other assets such as high-powered cars.
Museums are not the only places that have raised their guard.
Earlier this year, Britain widened a ban on the sale of rhino trophies, removing the right to sell those that dated before 1947, after an unusual increase in their price at auction houses in Europe.
Even taxidermists are on the lookout for suspicious activities.
The European Taxidermy Federation (ETF) sent out a letter to its members in early July warning them that Danish and Swedish taxidermists had been contacted by suspicious buyers claiming to be from Ireland or Britain.
The callers never say who they are and call from unregistered mobile phones, the federation's president, Vagn Reitz, wrote in the July 7 letter.
"All this stinks of illegal activity," Reitz wrote, "so it is a very good idea not to get involved if you are not 100 percent sure the trade is legal."