Two men wearing head torches climbed through a forced-open window at the Fondazione Magnani Rocca museum near Parma on the night of March ​22 and calmly lifted artworks by Cézanne, Renoir and Matisse from the walls.

Within three minutes, they had passed the works, valued at about €9 million ($10 million), to an accomplice and fled. At the museum's main gate, a fourth man discharged a fire extinguisher when the alarm sounded, distracting the custodian.

"Whatever could go wrong, did," said Stefano Roffi, director of the private foundation that runs the museum.

The raid was one of a string of museum thefts in France , Italy and Spain since last year's heist at the Louvre in Paris, raising concerns that criminals are increasingly targeting smaller institutions with fewer ⁠resources to protect valuable collections.

"This is where we see the 'Louvre effect'," said Marc Balcells, a Barcelona-based researcher specialising in crimes against cultural heritage.…