Usman Khan was released from prison in 2018 for plotting a terror attack. He'd undertaken two de-radicalisation programmes designed to turn him away from violent extremism. Yet despite efforts to rehabilitate him, Khan launched an attack near London Bridge - killing two people. It was the first of two violent attacks involving convicted extremists in a little over two months. So just how effective are schemes designed to de-radicalise offenders? For the first time, File on 4 hears from those at the heart of these programmes - the 'intervention providers' tasked with turning offenders away from violence. Some say offenders are able to cheat the system and convince the authorities they've changed their ways. So how can these intervention providers ever know when their work has been successful? The programme hears from a serving prisoner in a maximum security jail who says convicted terrorists are "gaming" the system by pretending to comply with "de-radicalisation" courses - and he warns that non terrorist offenders are being dangerously radicalised. Reporter: Adrian Goldberg Researcher: Luke Radcliff Producer: Helen Clifton Editor: Carl Johnston