BOGOTA, Colombia—The author of a letter claiming responsibility for the theft of a Goya engraving from the Gilberto Alzate Avendano Foundation in Bogota, Colombia, has admitted the letter was a joke, Deutsche Presse-Agentur reports via Colombian paper El Tiempo.

Lucas Ospina, an art professor at Los Andes University told the paper that his letter was "an artistic act." "Believing that letter is like believing in one of the imitations of 'La Luciernaga' [a radio program]," he said. "It would be like someone imitating the president and the next day the media reporting as statements by Uribe."

The Goya artwork in question, Tristes Presentimientos (Sad Premonitions), is part of the "Disasters of War" series and was stolen last Thursday from the foundation, where it was on loan from the Goya Museum in Fuendetodos, Spain, as part of an 80-piece show of the artist's work.

On Sunday, the foundation received a letter from the alleged thieves, calling themselves Commando Free Art S-11 and saying they were protesting the high price of tickets for the exhibition.

According to Ospina, the letter is a parody of a letter that guerilla group M-19 sent in 1974 when it stole the sword of Simon Bolivar. He originally posted it on an Internet art blog.

"It it important to clarify that the facetious text 'Goya, your engraving returns to the struggle' and its images were exclusively sent to the moderator of Public Sphere from my personal email, and that I clearly identified myself as Lucas Ospina," he said.

Police Major Jaime Alberto Escobar, chief of the division investigating the theft, said the team had so far uncovered $600,000 worth of silver pieces, but not the Goya. He added that he did not think the engraving had left the country.