Bogota - A valuable engraving by Spanish painter Francisco de Goya was stolen from a museum in Bogota, and the museum received a letter Sunday in which the thieves said they were protesting the high price of tickets for the art exhibition.

The work Tristes Presentimientos (Sad Premonitions) belonged to the series The Disasters of the War and was part of an 80-work exhibition on Goya (1746-1828) in the Colombian capital.

Police were looking into the letter regarding the robbery - signed by the so-called Commando Free Art S-11 - and analysing footage from security cameras.

Exhibition curator Luis Ernesto Ortiz asked citizens to assist authorities in finding the engraving. He called the theft a sad incident, which damaged international confidence in Colombia for future exhibitions.

The stolen work had been lent to the Gilberto Alzate Avendano Foundation in Bogota by the Goya Museum in the artist's native town, Fuendetodos, Spain. The exhibition opened September 2 and is set to run through October 20. (dpa)