An organised crime ring orchestrating raids on museums and auction houses – including the Fitzwilliam Museum in Cambridge – has been convicted.

Between January and April 2012, a total of five cases of robbery, burglary, theft and attempted theft occurred across four different venues: Durham University Oriental Museum, Norwich Castle Museum in Norfolk, Gorringes Auction House in East Sussex, and the Fitzwilliam Museum in Cambridge.

Security at the Fitzwilliam was tightened after the break-in in April 2012, during which £15 million worth of Chinese jade and rhinoceros horns were stolen. The incident was described by the former acting director of the museum as having tarnished the museum’s reputation as a “guardian of treasures”.

Local police launched their respective investigations, and a number of people were convicted for the crimes. However, it was soon apparent that the incidents were connected and organised by a criminal group that commissioned the jobs. Even though much of the stolen property has been recovered, several high-value items are still missing. Chief Constable Mick Creedon, leader of the Association of Chief Police Officers for organised crime, said that the conviction “follows a long and complex pan-European investigation involving officers from 26 police forces and the Serious Organised Crime Agency.

“The series of burglaries last year had a profound effect on museums and similar institutions and we are committed to bringing all those who were involved in the conspiracy to justice.

“Many of the stolen Chinese artefacts are still outstanding and a substantial reward remains on offer for information which leads to the safe return of those priceless items.”

In June 2012, supported by the National Crime Agency and National Police Chief’s Council, the Cambridgeshire and Durham Constabularies jointly launched ‘Operation Griffin’ to carry out an investigation into the conspiracy. A total of 14 men were charged in connection with all six incidents.

Last Friday, following an eight-week trial at Birmingham Crown Court, Daniel ‘Turkey’ O’Brien, 45, John ‘Kerry’ O’Brien, 26, Michael Hegarty, 43, and Richard ‘Kerry’ O’Brien Junior, 31, all from Cambridgeshire but with links to Rathkeale in Ireland, were found guilty of conspiracy to steal. Eight other men, aged between 33 and 68, from Cambridgeshire, London, Southend-on-Sea and Wolverhampton, were found guilty at three previous trials, all at Birmingham Crown Court. Two others, one aged 28 from Kent, and the other aged 46 from Belfast, pleaded guilty to the charges in March last year and January this year.

Senior Investigating Officer for the operation, Detective Superintendent Adrian Green from Durham Constabulary, said: “I am extremely pleased with the verdicts passed today and over the previous year.

“Because of the variations which can be given by auction houses the total value of the items targeted comes to anywhere between £18 million and £57 million. This illustrates just how massively profitable this trade was viewed by the gang.

“All the hard work put in by everyone involved has paid off. Firstly, those that carried out the burglaries were caught and convicted by local officers. What followed was a very long and complex investigation to capture and bring to justice those who commissioned and planned the jobs. I hope this sends out a message that nobody is untouchable.”