Theguardian_ Gold statues and jewellery made by the Italian sculptor Umberto Mastroianni have been stolen from an exhibition in northern Italy in a €1m (£850,000) heist.

The 20 gold statues and 30 pieces of jewellery were crafted between the 1950s and 1990s by the artist, who was the uncle of La Dolce Vita film star Marcello Mastroianni.

The items had been on display since December at the Museo d’Annunzio Segreto in Vittoriale degli Italiani, a hillside estate overlooking Lake Garda.

But on Thursday morning, a day before the show was due to close, managers found the exhibition space had been emptied of the works, which along with the statues included rings, bracelets, pins and brooches made by Mastroianni using the “golden stream” melding technique.

All but one of the pieces, which the thieves presumably dropped as they fled, were gone.

The theft was confirmed by Giordano Bruno Guerri, the president of the estate, which was the home of Gabriele D’Annunzio, a poet believed to have influenced Italian fascism.

“The good news is that a piece has already been recovered,” Guerri added in a statement. “Fortunately, nothing of D’Annunzio’s legacy has been touched.”

Italy’s art police are investigating the robbery, although as of Friday afternoon no developments had emerged. Guerri said updates would be provided at a press conference on Saturday.

According to a report in the local newspaper Brescia Today, the thieves entered the museum through a service door and deactivated the alarm.

“What happened at the Vittoriale is a very serious event,” Francesca Caruso, the councillor for culture in the Lombardy region, told the newspaper. “It is a cowardly gesture that has struck one of the highlights of Lombardy’s culture. I hope the works will be found as soon as possible and those responsible be brought to justice.”

Mastroianni, who died in 1998, was among the most famous Italian artists of the 20th century. He joined the Italian resistance during the second world war.