As any man or woman in police uniform will tell you, neutralising the terrorist threat of individual Islamist extremists is almost impossible.
Far-Right racist terrorists present their own problems, but they are generally easier to monitor because they tend to gather in places such as pubs and football grounds which can be more easily observed.
Lone wolf extremists, however, tend to keep to the fringes of society – both in the real world and online. And that can make them deadly.
Khairi Saadallah (pictured) is the man suspected of carrying out the horrific knife attack in Reading on Saturday which killed three people
That’s why the Libyan refugee Khairi Saadallah, suspected of carrying the weekend’s horrific knife attack in Reading, is every surveillance team’s nightmare.
Of course, he is still to be charged over these events and we are still to hear the reasons behind the killings. There will be a court hearing to determine these matters.
But what makes it doubly terrifying is that his background is sadly so common in parts of contemporary Britain.
I estimate that there are around 20,000 people of Middle Eastern and North African origin in this country who are regarded by MI5 as posing a potential terrorist threat.
Against this, MI5 employs some 4,000 people, and that includes cleaners and support staff. With numbers like that, it’s easy to see how the odds are not in the favour of law and order.
Many of these potential suspects are loners, disconnected from society by their experiences of violence in their homelands, or their drug habits.
Typically they have an unremarkable criminal record for theft or assault. Others are more directly menacing, and may be classed as having the potential to mount an attack like the one we saw in Reading at the weekend.
Of course, we are yet to pull together a complete portrait of Saadallah’s past. But I was hardly surprised to discover yesterday that he had already been convicted of a string of violent crimes.
Naturally, now that MP Angela Eagle said in Parliament that Saadallah was released from prison only 17 days ago, questions will need to be asked about why an Appeals Court judge thought it wise to cut his prison sentence.
Lavour MP Angela Eagle (pictured) said that Saadallah was released from prison only 17 days ago
But as someone who has served on the police frontline, I know fully well how tricky it is to snuff out terrorism before it takes place.
Once, police officers had Control Orders that forced terrorist suspects to be kept under house arrest, or physically moved out of their area if there was a threat they would engage with terrorism.
In 2011 they were replaced by so-called Terrorism Prevention and Investigation Measures, which allowed a suspect’s computers or mobile phones to be confiscated to prevent them from contacting other extremists.
But what both these experiments failed to account for is that it’s impossible to follow a lone wolf’s activities with total rigour.
GCHQ does its best to monitor extremist websites dedicated to terrorist content. But there is little they can do given that hundreds, possibly thousands, of UK-based Muslims access Islamic State beheading websites.
Moreover, lone wolves typically don’t work or socialise outside of their mosques. Nor does it help that, like most men in their twenties, the lone wolves are fuelled by testosterone and an overwhelming sense of alienation.
You might put an electronic tag on their ankle, but realistically this is not going to stop a young man with little to lose from walking into the street with a kitchen knife.
Even once a suspect is identified, keeping track of a single potential terrorist around the clock requires three or four surveillance teams of 15 officers each, so around 50 people are committed to that one suspect. Covering 20,000 suspects is therefore impossible.
Of course, that isn’t to say we should simply give up.
One area where the Government needs to be much more proactive is in its investigation of the bankrollers of new Muslim centres, some of whom are adherents of an extremist Wahhabist branch of Sunni Islam and encourage terrorism.
It is also imperative that we rethink our outdated attachment to a Dixon-of-Dock-Green-style of policing, and get our officers armed.
In fact, while every one of them is a tragedy, that so few lives have been lost to recent Islamist terrorism is merely good luck.
The London Bridge attack last year was mounted just across the river from the City of London’s armed response unit, which was quickly on the scene to neutralise the terrorists.
As for the Reading attack, it was only brought to an end by a passing policeman who heroically rugby tackled the suspect.
Saadallah’s attack on Saturday was only brought to an end by a heroic policeman passing by who rugby tackled the suspect in heroic fashion
With our police unarmed, what keeps me awake at night is the prospect of a coordinated attack in a remote area, many miles away from an armed response unit.
We cannot afford to have a situation where the first response to an attack is from unarmed police who might be forced to pull back until an armed unit makes its way from the other side of the county while the terrorists continue their rampage.
I say this not as a retired police officer, but as a member of the public: when I go shopping or out for dinner I want to know there are armed police in reasonable range should one of these now-familiar attacks unfold around me.
Those who hanker after that quaint postwar era of policing forget that in The Blue Lamp, the film that inspired the BBC series Dixon of Dock Green, PC Dixon was actually shot dead by a hoodlum played by Dirk Bogarde.
Fast forward to today, and it seems we’re yet to learn the lessons of having an unarmed constabulary.
Kevin Hurley is a former Surrey Police and Crime Commissioner and Detective Chief Superintendent in the Metropolitan Police