Detective Superintendent Stephen Ratcliffe said this was "a gang of experienced criminals who had been the subject of an investigation by the special projects team at the Metropolitan Police for a couple of months before this robbery."
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Paramedics were on standby outside the police cordon as armed officers stood by the property.
A team of researchers has taken samples from the ten busiest London Underground lines to work out which are the dirtiest.
The action comes on top of an emergency timetable which has seen hundreds of trains a day being axed.