Police in Spain have made seven arrests in connection with the theft of five paintings by the late Irish-born artist Francis Bacon worth €25m (£19m), which were stolen from a house in Madrid. None of the paintings have been recovered.

The theft took place at an apartment in Madrid in July 2015, but was only reported by the media in March 2016. The theft was carefully planned and involved disabling an alarm system and tracking the movements of the owner to ensure he did not return and catch the thieves red-handed. The thieves left evidence behind them and also took personal belongings including jewellery and coin collections from the owner.

Francis Bacon (1909-1992) was a regular visitor to Madrid where he studied the old masters at the Prado Museum. He was one of the world's leading artists and in 2013 his Three Studies of Lucien Freud was sold at auction for a then world-record price of $142.4m (£90m).

When news of the theft became public experts warned it would be almost impossible to sell on the paintings due to the fame of the artist. One expert speaking on condition of anonymity told a newspaper: "It is not at all easy to sell a Francis Bacon, large or small, without that getting to the ears of those who pore over such a rarefied sector."

The arrests were made as a result of using specialised forensic methods to analyse photographic imagery. British investigators – who specialise in recovering stolen artwork – received an email with pictures of the missing paintings asking if they were stolen, according to The Guardian.

Using metadata – information stored in digital photos, which can include the time, date and location and the camera used to capture the photo – experts were able to identify the camera used to photograph the paintings and trace the shop it was rented from.

The owner was then able to provide information about the person who had rented it. The photographer is thought to be among those arrested in Sitges, northern Spain. A Madrid art dealer and his son are also thought to be among those being questioned by police.

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