City of London Police say anyone with information should call 020 7601 2222, or Crimestoppers, anonymously, on 0800 555 111.
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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.