A security camera that filmed the three men arriving and leaving the Graff workshop in Hatton Garden showed them letting themselves in. They then quickly got through three combination-locked internal doors.
Detectives are still questioning staff at Graff as it becomes clear that the gang who carried out Britain's biggest raid on a jeweller's had inside information.
Detective Inspector Robin Berger, of the Flying Squad, the officer in charge of the inquiry, said yesterday: 'It is obvious to anybody that there was a high degree of preparation. They knew the premises very well.
'There are a number of members of staff being questioned and there are a number of people associated who will be helping us with our inquiries.'
The robbers also knew that there were security cameras in the entrance hall and shielded their faces as they passed them. They were all wearing baseball caps, which had concealed stocking masks that they pulled down before entering.
The men, all white, well-built and 5ft 10ins to 6ft tall, went straight to the sorting room at the workshop where jewellery is made for Graff's shops in the West End. The firm's customers include some of the world's wealthiest people.
There they threatened Raymond Graff, brother of the company's owner Laurence, his son and two employees with handguns and handcuffed them before emptying the safe. The most valuable part of their haul was a 25-carat, heart-shaped diamond worth pounds 1.5m. Det Insp Berger said: 'They had been hanging around prior to the robbery. They appear to have gone in within two minutes of Mr Graff and his son arriving.'
One of the men had a beer gut and was wearing a light shirt and a thigh-length jacket. The second was thick-set with dark hair and carrying a white plastic bag, while the third wore a light- coloured top with a dark rim around the neck and chest area.
Police are appealing for anyone who was in Hatton Garden when the men arrived at 8.40am on Thursday, and left four minutes later, and who saw anything, to telephone them on 071- 230 2061.Reuse content