investigation or is being called on for expertise by desk officers.

They are a pre-arrest unit, with access to court records.

Lone actors

The past few years have seen the emergence of a particularly tough target to detect: the lone actor, more commonly referred to as "the lone wolf".

Behavioural scientists assess this term as meaning "someone who undertakes an attack without being part of a network".

So would that apply say, to the Tunisian mass murderer Mohamed Lahouaiej-Bouhlel, who crushed 84 people to death with his lorry in Nice in July?

Image copyright Kapitalis via Associated Press Image caption Mohamed Lahouaiej Bouhlel killed 84 people in Nice on 14 July 2016

Yes, it probably would, even though so-called Islamic State later claimed he was one of their "soldiers of the caliphate".

In its online magazineswith personality disorders or other mental health issues.

The dyad

There is also a new term that has crept into the lexicon of counter-terrorism: the dyad.

This refers to a pair of individuals working together on a plot, such as the men who murdered Fusilier Lee Rigby in Woolwich in 2013: Michael Adebolajo and Michael Adebowale.

Image copyright Metropolitan Police Image caption Lee Rigby's killers, Michael Adebolajo and Michael Adebowale, were a "dyad"

Dyads are thought to be of great concern because their ideas feed off each other, reinforcing their beliefs and lending confidence to their plans.

That makes them harder for MI5 and the police to detect as they don't "leak" clues to observers so readily.

"Protective risk factors"are the inhibitors, the things that stop a person from actually going through with an attack.