(Wine Spectator) - Three years ago, Peter Sisseck grew suspicious when a cork supplier his Ribera del Duero winery, Dominio de Pingus, didn't normally use contacted his office about a request for Pingus-branded corks placed by a restaurant in Galicia.

"We sort of let it go," Sisseck told Wine Spectator.

But two years later, a Danish buyer complained about two seemingly fake bottles of Pingus he had purchased at auction in Belgium. When Sisseck looked into it, he found out the wine had been supplied by someone from a restaurant in Galicia. "It struck me that when we investigated the incident with the corks, it also came from a restaurant in Galicia," said Sisseck.

His suspicions sparked an investigation by Spain's Civil Guard. And last month, officers announced that they had broken up a counterfeiting ring that had been operating for four years, allegedly producing fake bottles of collectible Spanish wines, like Pingus and Bodegas Vega Sicilia. Since the probe began June 18, officers have arrested four people in connection with the allegations and are investigating four more. The suspects face charges ranging from money laundering to crimes against industrial property and public health.