The central aim of the BSU is to flag any potential threats; they are mainly looking for those people, who will boast about or are ready to carry out an attack.
The people who are selected for surveillance tend to be chosen based on intelligence gathered from the agency's network of informants as well as from information from the public.
"Every saint has a past, and every sinner has a future" — scribbled on a whiteboard in the office of MI5's Behavioural Science Unit
— Chris Schofield (@chrischofield1) August 9, 2016
The experts then search for signs of unusual activity, such as an increasing sense of grievance, a desire to acquire skills and tactics — an attempt to identify materials for their plans and logistical practice and trial runs.
According to Neil, an Arabic and Norwegian speaker, who has worked for the unit for six years, it takes a lot of guts to talk about carrying out an attack and actually doing it.
"It takes some doing to go from talking about carrying out a violent act like killing to actually doing it," Neil said.
Research by MI5 shows that more than 60 percent of attackers provide some clues that they are about to do something, and this is comes from the changes in their behaviour.
An example of this came from Roshonara Choudhry, who was inspired by Al-Qaeda. She showed signs of change in her life before stabbing MP Stephen Timms in 2010.
MPs at risk.
— Vetrissimo (@vetrissimo) 18 June 2016
14 May 2010 Muslim Roshonara Choudhry stabbed British MP Stephen Timms in his constituency surgery. pic.twitter.com/rylMwrzIii
Choudhry quit her college course, cleared out her bank accounts to stop authorities from having access to her money, and settled her student loan, fearing her parents would be liable for the debt, before carrying out the attack.
"We deal with probabilities and that is the nature of our work. We provide an assessment of the subject of interest but the final decision lies with the [investigating] officer," Neil said.
"When it comes to lone actors, we have to question why they have chosen to act alone. Is it because it is more operationally effective? Is it because others don't want to be involved in their plot? Is it because they cannot relate to others or others find them to obnoxious to work with them?"
Neil also said that the assumption that many extremists have mental health problems is wrong.
"Only two percent of members of terrorist organizations suffer from mental health problems, compared with an average of up to 30 percent of members of the public," he said.
The BSU and other security services are monitoring hundreds of suspected British jihadists, including 400 who have only just recently returned from Syria.
