The role the internet played in the radicalisation of 437 convicted offenders in England and Wales was examined by academics and published in a study by the UK's Ministry of Justice.
The study found that the internet is “increasingly prominent” in radicalisation but plots from attackers radicalised online were “most likely” to be foiled.
The proportion of prisoners holding Islamist extremist views has fallen to its lowest level on record while the number of those categorised as adhering to an extreme right-wing ideology has risen to its highest level in the past year, the figures also reveal.
A third of the sample of criminals surveyed in the research had mental health problems or personality disorders.
Countries on global terror index — worst — in pictures
The conditions most commonly reported included Autism Spectrum Condition (ASC) and depression, with these “most common” among those mainly radicalised online, the findings indicated.
Analysis of specialist reports from 2010 to the end of last year also suggested the biggest increase in online radicalisation over time was among female offenders and people aged over 25, researchers said.
“Technological advances have led to changes in the types of applications/platforms used over time.”
The report added: “For attackers specifically, those exposed to online influences in their radicalisation pathway were more likely to use the online domain for attack-planning behaviours.
“Those attackers reported as being primarily radicalised online were found to be the least successful in plotting attacks and most likely to see their plots foiled at the planning stage.”
Countries on global terror index — best — in pictures
The research was carried out by Nottingham Trent and Bournemouth universities with the Prison and Probation Service and follows on from a report published last year.
Lead author Jonathan Kenyon said the study provided a “contemporary picture” of the online activities of convicted extremists in England and Wales until the end of 2021 and found “marked differences” in behaviour and offending between those who were radicalised on the internet, in person or a mixture of both.
This highlights the importance of taking these factors into account when “assessing risk” and considering how to tackle terrorism, he said.
Jens Binder, associate professor of psychology at Nottingham Trent University's School of Social Sciences, said mainstream websites and apps were “routinely” used, “sometimes to reach out to the many users there and to lead some of them to more secluded online locations”, which is “likely to require a more proactive and transparent approach from tech companies” so radical content is reported.
UK increases terrorism threat level to 'severe' — video
Christopher Baker-Beall, senior lecturer in crisis and disaster management at the Bournemouth University Disaster Management Centre, stressed the findings were “not suggesting that those with mental illness represent a community from which terrorists are more likely to originate”.
“Nor does the report suggest that mental illness be viewed as a predictor of terrorist intent,” he continued.
“Instead, it highlights the importance of providing mental health support to those convicted of extremist offences to ensure they do not go on to reoffend or commit further acts of terrorism”.
The Ministry of Justice said the views expressed in the report are those of the authors and “are not necessarily shared” by the department and added: “Nor do they represent government policy”.
Last month, MI5 director general Ken McCallum described extreme right-wing terrorism as now a “diffuse online threat”.
“From the comfort of their bedrooms, individuals are easily able to access right-wing extremist spaces, network with each other and move towards a radical mindset,” he said.