Detective Chief Inspector Harry Hennigan has defended the decision by authorities to let a man out on day release from prison, who subsequently carried out a series of armed raids.
Sean Bradish was handed down two life sentences having already received one previously.
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The mayor said a post-Brexit visa system was needed to maintain the supply of skilled migrant workers for London's economy.
Peter Barnett made hundreds of trips from his home in Oxfordshire to Marylebone station without paying the full fare.
Police say he may have attacked more children and urged them to come forward.