Conspiracy theories are rife in the wake of what has been described as “one of the most significant art thefts ever to happen in Italy”.

The heist took place on Thursday night (19th November) when three armed and masked men raided the galleries of Verona’s Museo Civico di Castelvecchio, They escaped with 17 paintings with a total value of around €15 million (£10.5 million), 11 of them masterworks.

Verona council spokesman Roberto Bolis described how the thieves seized on a window of opportunity between the time the museum closed to the public but before the security alarms had been activated. After tying up and gagging the only private security guard in attendance as well as a cashier, one thief watched over the hostages while the other two went from room to room removing the paintings.

The bandits also took the guard’s keys and used his car to get away before switching to another vehicle. Some paintings were removed from their frames, others were left intact.

Corriere Della Sera listed the stolen works as followed:

“Madonna and Child” or “Our Lady of the Quail”, by Antonio Pisano, better known as Pisanello

“St. Jerome Doing Penitence” by Jacopo Bellini

“The Holy Family with a Saint”, by Andrea Mantegna

“Portrait of a Child with a Drawing” and “Portrait of a Young Benedictine”, by Giovanni Francesco Caroto

“Madonna and Child”, “Transport of the Ark of the Covenant”, “Belshazzar’s Feast”, “Samson” and “Judgement of Solomon”, by Jacopo Tintoretto

“Portrait of a Man”, by the circle of Jacopo Tintoretto

“Portrait of a Venetian Admiral”, by Domenico Tintoretto

“Portrait of a Venetian Admiral”, from the studio of Domenico Tintoretto

“The Lady of Licnidi”, by Peter Paul Rubens

“Landscape” and “Seaport”, by Hans de Jode

Investigations are being led by a specialist culture heritage unit of Italy’s paramilitary police force, the Carabinieri, to uncover the mastermind behind the theft. Meanwhile, local authorities, art critics and historians have been speculating on how such a devastating loss could be wrought on the museum’s treasured collection.

The carefully planned and skilfully executed nature of the heist has led some to believe the theft may have been specially commissioned. Mayor Flavio Tosi cited the slick nature of the operation as a reason to believe it was perpetrated by a private collector. “The thieves were well timed, well organised and knew what they wanted”, he said.

This theory is also said to be supported by the high-profile nature of the works, which would make them impossible to sell on the open market. Art historian Tomaso Montanari compared the incident to breaking into the Uffizi Gallery Museum in Florence and stealing a Botticelli, an iconic work that would immediately arouse suspicion if it was consigned for sale.

A more radical theory suggests the theft may have been carried out with a view to selling the works on the black market for the benefit of ISIS. At the very least, former junior culture minister and art critic, Vittorio Sgarbi, believes it may have been a demonstrative act by the terrorist organisation:

“Only an idiot would steal such similar paintings which are unsellable, and this makes you think that it was a theft to ask for a ransom or a jihadist act.”

Others believe the story behind Thursday’s raid is much more simple. As deputy head of the Carabinieri, Alberto Deregibus told Reuters:

“It may have just been delinquents who thought: ‘Let’s steal them and decide later what to do with them.”

Whatever the circumstances, the theft has shocked and saddened Italian authorities. Although the incident is one in a spate of thefts to have struck European museums in recent months (including the theft of a £40,000 Elisabeth Frink statute from London’s Beaux Arts gallery in July) the number of heists has actually been in decline in Italy, dropping by a quarter between 2012 and 2013.

Montanari told la Repubblica he was baffled by the fact that the museum was protected by “a single, private armed guard, like a supermarket”. He believes the fault lies with the government, which has slashed spending on cultural institutions and he called for tougher sentences for art thieves. Mayor Tosi has defended the museum’s existing security measures.

One of the most important museums in Verona, the Castelvecchio Museum is housed in a castle built by Cangrande della Scala in 1354 and renovated by architect Carlo Scarpa between 1958 and 1974. Its collection of early Christian artwork, 10th-14th century sculpture, medieval arms and armour and 14th-18th century paintings is monitored via 48 security cameras.