Two armed men have stolen eight engravings by French artist Matisse and at least another five by Brazilian painter Cândido Portinari from a library in São Paulo.
Brazilian officials say the thieves held up a security guard and an elderly couple who were visiting the library before making off with the artworks on foot.
The robbers reportedly entered the library by the main entrance at 10:00 AM local time on Sunday, and left by the same route, heading towards the nearest metro station.
This heist comes less than two months after the art world was rocked by a brazen break-in at the Louvre museum in Paris, where thieves made off with priceless jewels.
The engravings stolen from Biblioteca Mário de Andrade on Sunday formed part of a joint exhibition with the São Paulo Museum of Modern Art.
The thieves targeted the exhibition, entitled From Book to Museum, on its final day.
Biblioteca Mário de Andrade is the country's second largest library, and officials say that its building in the center of São Paulo is equipped with cameras that use facial recognition technology.
The mayor of São Paulo reported that the thieves have already been identified but remain at large.
Officials have yet to release a detailed list of the stolen works, but sources revealed that a collage Matisse created for the art book Jazz was among the pieces taken. Matisse is widely regarded as one of the 20th Century's most influential artists, and critics have deemed the value of the stolen works as incalculable.
The thieves also took at least five engravings by Portinari, which were created to illustrate a special edition of the novel Menino de Engenho (Plantation Boy) by Brazilian writer José Lins do Rego. Portinari is noted for his depictions of rural workers and is considered a significant figure in Brazilian Modernism.
