MADRID — Spanish tax inspectors checking the contents of a safety deposit box discovered a painting believed to be by Dutch master Vincent Van Gogh that went missing almost 40 years ago, a tax office source said on Saturday.

Confirming a report by Spanish newspaper El Mundo, the source said the painting was entitled "Cypress, Sky and Country" in English translation from Spanish and dated 1889. The pastoral work had last been on view in the Kunsthistorisches Museum in the Austrian capital Vienna, the source told Reuters.

He said an investigation was under way into how the painting wound up in the safety deposit box and whether it disappeared as a result of a robbery.

Art experts told the tax office that the painting, bearing three seals of authenticity and measuring 13.7 by 12.5 inches, is most likely genuine although this must still be confirmed by Spain's Culture Ministry. The value of the painting has not yet been assessed.

Van Gogh, a giant of the impressionist school whose works are among the most valuable in the world, committed suicide penniless in 1890 after a long battle with mental illness.

The painting was found during an operation to seize the contents of some 542 safety boxes from tax offenders who owed around 319 million euros — about $438.83 million, El Mundo said.