Forensic clues: Blood specks in Yorkshire warehouse converted into drug factory told murder tale

The drug factory had been set up after a triad began renting unit 12 on the Elvington Industrial Estate, near York, under the pretence of running an oriental cash and carry warehouse.

The criminal gang behind the operation spent up to 80,000 converting one side of the industrial unit into the cannabis farm, hiding it behind false floors and

walls while storing some food to maintain the illusion of a warehouse.

The inside walls were heavily insulated to prevent the warmth generated from lighting and heaters being picked up by infra-red equipment and accommodation was created for workers.

North Yorkshire Police's Organised Crime Unit executed a search warrant at the factory on February 25 last year after covert surveillance launched following a tip-off and found a major skunk cannabis farm with hundreds of plants worth more than 300,000.

But at that stage no one knew the factory was also a murder scene. A murder inquiry was only launched after a body was found in a canal the following month and links to the cannabis farm in Elvington emerged.

A photograph of Mr Chen with three dates of birth scrawled on the back was discovered during a further police raid on a house in Heslington Lane in York – and the picture proved key to formally identifying the body.

Detectives had drawn up a list of missing Chinese men using the name of Bing Ling – the false name which Mr Chen had employed – and they were able to marry up one of the dates of birth and obtain a fingerprint match.

Forensic examination of the building in Elvington using specialist chemical treatments revealed tiny traces of blood on a plasterboard wall, which that proved to be Mr Chen's.