The robbers used a mini-bulldozer to smash through a locked door to get into the building, then put on gas masks as they detonated smoke bombs. Sledgehammers and a nail gun were inadequate against bullet-proof glass, police said. Nor had the thieves expected more than 100 police officers to be waiting, many of them disguised as cleaners with weapons concealed in rubbish bags. The four men inside were soon overpowered by police and arrested in an operation codenamed Magician. Officers who had cordoned off nearby streets swooped on several other men and nabbed another waiting in the high-powered boat.
Police believe the gang had got close to staging the raid at least a dozen times.
They alerted De Beers about five weeks ago, and the gems were replaced with crystal lookalikes on Monday. Investigators say they are keen to learn more about the gang's plans for the gems.