The photo of a Grégory Villemin, a child assassinated in a celebrated case at the age of four in 1984, was used to advertise the “kindergarten” in the festival’s daily publication, Montreux Jazz Chronicle, on Saturday.

The murder of the child, who was found in a river in the Vosges region of France after his parents received anonymous threatening letters, has never been solved.

The embarrassing use of the photo was spotted by someone who informed the right-wing National Front party, whose wing in Boulogne-Billancourt published the news on its website.

The information subsequently found its way onto Twitter, with an internet user noting that the electronic version of the Montreux publication did not contain the same image for the kindergarten ad used in the paper version.

Contacted by Le Huffington Post, the French-language online news site on Tuesday, Mathieu Jaton, managing director of the Swiss music festival, said the use of the photo was a regrettable “human error”.

The image was downloaded by a “very young person who did not know of the (child murder) case,” Jaton said.

He assured that it was not a “bad joke” but rather a convergence of “stupid circumstances”.

Jaton said the error was noticed too late to change the paper version of the Montreux Jazz Chronicle but the electronic version was immediately modified.

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Thierry Moser, a lawyer for the parents of the murdered boy, said they were scandalized by what happened.

“I invite organizers to to send me a letter of apologies in good and due form,” Moser said in a release published by French and Swiss news media. 

The Montreux Jazz Festival, one of Switzerland's best known summer music festivals, held in the lakeside town overlooking Lake Geneva, runs until July 20th.