In a scene reminiscent of a Hollywood thriller, the thieves raided a safe-deposit company in central London.
The gang used the calm of the Easter holiday weekend to tear through the vault at Hatton Garden Safe Deposit Ltd before making off with the contents of around 300 safe deposit boxes, used by the area's jewellers to store their stock.
The robbers are thought to have clambered down or abseiled through a lift shaft, with detectives investigating whether they were already inside the Hatton Garden Safe Deposit company when an alarm went off on Good Friday.
Officers confirmed that the gang used heavy cutting equipment to get through the vault's 18in-thick door.
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A source at the safe deposit company told 'The Times' that a member of staff checked the alarm on Friday but gave the all-clear when the front and back doors of the building appeared to be secure.
It is understood that the main caretaker, who would have been likely to call the police, was not informed about the alarm.
Among the most valuable items feared to have been stolen is a half-cut aqua diamond worth 500,000 pounds.