A self-portrait painted in 1926 by the Italian Modern master Giorgio de Chirico was stolen from the collection of the Béziers Museum of Fine Arts in Southern France last week.

The painting in question, Composition With Self-Portrait, first inventoried in 2015, has been described as “priceless” by local prosecutors. It was part of the collection of Jean Moulin, the French Resistance hero who was a Béziers native and an avid fan of Modern art.

The oil painting was cut from its frame last Thursday afternoon at the Hôtel Fabrégat, which houses some of the museum’s collection, including Jean Moulin’s private holdings, gifted to the city by Moulin’s sister in 1975, and which also includes works by Soutine, Raoul Dufy, and Suzanne Valadon.

The art museum does not have any security cameras. The empty frame was noticed shortly after the theft by museum guards, according to the local city council. An official complaint was filed by the city that same evening, although the news of the theft was not publicly revealed until yesterday.

The 20th-century Surrealist painter’s work has sold for millions in the past. Il Ritornante (1918) notably sold at Christie’s Paris in 2009 for over €11 million—a record for the artist—as part of the auction house’s sale of the collection of Yves Saint Laurent and Pierre Bergé.

The stolen self-portrait, “because of its prominence, is not tradable on the art market in France or internationally,” wrote the council in their announcement.

The work was intended to be exhibited in the future museum of the birthplace of Jean Moulin, which the council said was a much more secure location.

An investigation into the theft is being carried out by the Montpellier regional police. Police commissioner Anthony De Freitas confirmed the regional judicial police service was appointed by the Béziers prosecutor.