Last week in Paris, four or five masked men entered a Harry Winston store and committed one of the largest jewelry thefts ever.

A brazen and meticulously planned robbery of the Harry Winston store in central Paris last weekend netted the unknown thieves about €20 million, or $28.4 million, in gems, one of the largest jewelry thefts ever, French investigators said Thursday, following an inventory of the raided safe.

The thieves are now probably attempting to sell the stolen goods on the black market to receive a payout. Traditionally, stolen goods are unable to fetch the full market price for obvious reasons. According to the article, thieves can receive a little as 20 percent of the value of the stolen good.

“These guys are professionals. Beginners don’t dare go near top-end jewelers,” the official said, adding that the real test of the gang’s acumen will be whether they can resell the stolen gems on the black market.

Because of the risk of detection, stolen jewels tend to fetch only a fraction of their retail price, sometimes as little as 20 percent, analysts say. French investigators have been circulating information about the stolen merchandise to the police in other countries in the hope of catching the thieves when they approach a prospective buyer.