French police raided the storage rooms of storied French auction house Drouot and swept through the homes of several employees on Wednesday after a missing painting by Gustave Courbet was discovered at the residence of one of the house’s employees, leading to the discovery of other missing paintings.

The painting that prompted the investigation, The Wave, was reported stolen in 2004. Valued at as much as £900,000 ($1.3 million), its disappearance led to the formal judicial inquiry, which launched Wednesday’s action.

In all, 12 people — including an auctioneer, eight commission agents, and three related family members — were detained over the course of the day, though police from the agency charged with combating art trafficking declined to provide further details, citing an ongoing investigation.

Read more at the Associated Press.