Five minutes to steal a Renoir: Why museum thefts are becoming bolder across Europe

Museums are vulnerable and easy to burgle, Brand says, compared to well-secured jewellery shops and banks - which these days hold less cash.

Directors historically "never have enough money for security" or prefer to spend it on the next exhibition, and museums are often housed in old buildings that are difficult to protect, he adds.

For example, the Denis Diderot Maison des Lumières (House of Enlightenment) - burgled hours after the Louvre - is housed in two 18th-century mansions in Langres.

"We therefore have to reconcile modern security requirements with the need to preserve historic buildings and comply with France's heritage regulations," local mayor Théo Caviezel explains.

Museums are also open public spaces that have less access control and perpetrators can come in as visitors, says Kumar Sokka, CEO of Acre Security which works with some museums in Europe.

And even if a museum is secured well, thieves could still come in armed, Brand says, adding that it would be impractical to put police in galleries.

Many of these burglaries are over in minutes and the thieves have long gone by the time police show up - the Renoir case happened within five minutes.

Heists used to depend on deception, but burglaries are now becoming increasingly violent, Europol's report last month, external points out. Offenders are using tools such as sledgehammers, axes, crowbars and, in some instances, explosives, to get inside.

In the Louvre, the thieves used a mechanical lift to gain access, cut through the window with power tools and then threatened guards, who left, before forcing their way inside displays and grabbing the jewels. In the Renoir case, the perpetrators used an electric knife to cut through a fence before sawing through structures holding the paintings' frames in place.

The increased aggression creates new security concerns.

"An armed incident during opening hours, like the one we experienced, creates a situation in which the immediate priority is to keep our audience and staff safe," says Lilli Hollein, the director of the Museum of Applied Arts (MAK) in Vienna, which was burgled last month.