AMSTERDAM -- Thieves broke into a Rotterdam museum on Tuesday and walked off with works from Picasso, Monet, Gauguin and Matisse potentially worth hundreds of millions.

Police haven't said how they pulled off the early hours heist, but an expert who tracks stolen art said the robbers clearly knew what they were after.

The heist at the Kunsthal museum is one of the largest in years in the Netherlands, and is a stunning blow for the private Triton Foundation collection, which was being exhibited publicly as a group for the first time.

"It's every museum director's worst nightmare," said Kunsthal director Emily Ansenk.

The stolen paintings were Pablo Picasso's 1971 "Harlequin Head"; Claude Monet's 1901 "Waterloo Bridge, London" and "Charing Cross Bridge, London"; Henri Matisse's 1919 "Reading Girl in White and Yellow"; Paul Gauguin's 1898 "Girl in Front of Open Window"; Meyer de Haan's "Self-Portrait," around 1890, and Lucian Freud's 2002 work "Woman with Eyes Closed."