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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Thousands of passengers now face delays, with Southeastern trains warning "all services" would be affected.
Paramedics attended a scene in Willesden, north-west London, on Monday afternoon where they found the teenager with stab wounds.
At least five people have suffered minor injuries but the cause of the blast is still unknown.