Romanian authorities have arrested three men suspected in last year's heist in the Netherlands at a Rotterdam art gallery, where paintings ranging from artists like Pablo Picasso to Claude Monet were on display.
Police said the paintings taken had not been recovered but indicated that some of the works might be hidden in an undisclosed location in Romania.
Seven works of "considerable value" disappeared in the museum theft, spokeswoman Mariette Maaskant said on Netherlands public radio.
"Initial investigations show the burglar was well prepared," police said in a statement.
Among the paintings taken were Picasso's "Tete d'Arlequin," Henri Matisse's "La Liseuse en Blanc et Jaune," and Monet's "Waterloo Bridge, London" and "Charing Cross Bridge, London," Rotterdam police said.
Also taken were Paul Gauguin's "Femme devant une fenêtre ouverte, dite la Fiancee," Meyer de Haan's "Autoportrait" and Lucian Freud's "Woman with Eyes Closed."
The Kunsthal museum's alarm system went off shortly after 3 a.m. local time on a mid-October morning, alerting the exhibition hall's private security detail. When security staffers arrived by car, they saw that the paintings were missing, Rotterdam police spokesman Roland Ekkers said.
The works belong to a private collection that is being shown for the first time to the public, according to a Kunsthal statement.
The Triton Collection has taken 20 years to assemble and includes more than 150 works of modern art from the "late nineteenth century to the present day." It spans art movements from impressionism and expressionism to cubism and constructivism.