One of Britain’s top psychologists has warned that it can never be certain that terrorists can be “cured” in any deradicalisation programme.
Christopher Dean said some terror offenders who take part in his Healthy Identity Intervention (HII) scheme appear to regress due to complex reasons such as who they mix with.
Mr Dean's comments come after HII participant Usman Khan stabbed two people to death near London Bridge on 29 November.
Khan was a convicted terrorist who had been a member of an al Qaida-inspired group that plotted to blow up the London Stock Exchange.
The HII scheme involves offenders like Khan attending sessions with a psychologist who encourage them to talk about their motivations, beliefs, identity and relationship with society.
Former senior Home Office official Ian Acheson said attention was drawn to shortcomings of the HII programme in 2016.
Speaking on BBC Radio 4's Today programme, Mr Dean said individuals can both progress and regress.
"Sometimes people move up two rungs, sometimes individuals may say I've had my doubts about this or that and they may be willing to speak to people, but equally they may go down rungs as well.
"They may come into contact with individuals, they may go through a spell in life where they may feel let's say aggrieved again, where they may begin to re-engage with groups or causes or ideologies associated with their offending behaviour.”
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Mr Dean said some offenders he worked with needed 20 or more sessions to show signs of positive change.
He added: "We see some individuals who may have been part of a group for many years or have been invested or identified with the cause for many years. [Leaving that group] is an incredibly difficult thing to do.”
Warning that there is no guarantee of success, Mr Dean said: "I don't think you can ever be sure that that's occurred. I think you can get increasing evidence over time and particularly behaviourally when people begin to behave in different ways and that's consistent over time and in different places.
"People can get more reassured and confident about change and progress that people are making, but yeah, I think we have to be very careful about saying someone has totally changed or has been cured.”
Mr Dean added that he would have a "healthy scepticism" of the notion that there is a perfect system to deradicalise terrorists.
He said: "I think we need to be careful about suggesting that interventions in themselves are the solution or the only solution or psychology is, but I think it's about continuing to work together in our learning…
"I think we must accept that there may be a small number of people who are potentially ideologically bulletproof and do not wish to recant their hateful views and we must actually start looking at them through the lens of public protection and national security rather than perhaps create unrealistic expectations of rehabilitation.”
Khan killed two people and injured three others in a knife rampage before being shot dead by police in November.
He had been released from prison on licence in December 2018, by which time Khan reportedly appeared to be responding to rehabilitation.