The greatest weapon police have against warped killers like Salman Abedi is the Muslim community itself, writes former RUC Special Branch detective Dr William Matchett.
A small constabulary is larger than MI5. It is unrealistic to expect Britain's main intelligence agency to follow every lead, monitor every suspect and stop every attack. Even for Northern Ireland, 3% of the UK population, it is impossible to thoroughly research everyone who expresses "Up the 'RA"-type support for active terrorist groups.
The men and women of Thames House are the UK's top intelligence experts. It is not, however, a law-enforcement organisation, but a part of the civil service. Its remit is to protect national security. And, in my view, it is the best in the world.
Although MI5 runs informers and conducts surveillance, it also excels in strategy, policy-making and organisational structure when it comes to guiding the National Crime Agency, police services and other branches of law-enforcement on intelligence.
Jack Morton, a former MI5 chief, was the father of the Royal Ulster Constabulary's Special Branch. He had served in many foreign hot-spots. People like him shaped an intelligence apparatus that defeated the IRA.
They knew what was needed to counter a violent threat and how to integrate MI5 with a constabulary. This compensated for MI5's limited operational reach.
Morton's successors also know what is required to combat the current crisis. In many ways, this is different to the IRA terrorist campaign. But, in many ways, it is the same.
The most obvious overlap is a militant ideology based on ancient hatreds and modern grievances. This persuades its followers that murder is a legitimate act. In their eyes, killing will stop the unlawful occupation of a territory.
Where bombs exploding in England delighted the IRA and their supporters, the same applies to groups like Isis and al Qaida. They get their message out by attacks that shock. Success is getting headline news.
At times, there is a cell structure and a network. With this, you have an opportunity to intercept communications, recruit an informer, or run surveillance. But for an individual who has self-radicalised and shown no real capacity to carry out an attack, it is almost impossible to stop.
In Manchester, it appears the bomber, Salman Abedi, was not alone. This is unsurprising. Bomb-makers are a precious resource. They are not wasted as martyrdom canon fodder.
In combating a diverse terrorist threat today, the main challenge for MI5 is a criminal justice system that routinely leaks secrets, modern policing and society's confessional culture.
Disclosing sensitive methodologies in open court makes it harder to catch terrorists in the future. This has been a source of frustration for MI5, as it was for the RUC Special Branch. An investigative-led police model aggravates the issue.
In an investigative-led approach - the norm in the UK - senior officers, like detective superintendent Stella Gibson in The Fall crime drama, call the shots. Intelligence supports the investigation. Which is the way it should be.
Getting forensic and witness evidence is the priority. This - and not intelligence - is what proves guilt beyond reasonable doubt.
Of confessional culture, one needs to look no further than the New York Times publishing details about the explosive device Abedi detonated. It was an unsolicited disclosure that angered the investigating team. Today's society is fixated with revealing everything.
Towards the end of the Troubles, as the investigative-led approach started to replace an intelligence-led approach, it was not uncommon to read a sensitive piece of intelligence in a national newspaper.
Secrecy is a knight in an age where transparency is king. For MI5, this poses problems.
I have lots of Muslim friends. They are decent people, who raise their kids right and love this country. Quite a few have seen the horrors of conflict up close and have had family members murdered by terrorists. They are outraged by Manchester. For the police, this is a great baseline.
The vast majority of British Muslims support the rule of law. In Muslim countries where I have worked, a minority of hate-filled zealots label the rest non-Muslims for criticising them. Violent extremists are bullies. They despise and demonise those who stand up to them.
After the Westminster Bridge incident, the Muslim community held a vigil for the five victims. The wife of Khalid Masood, the attacker, condemned his actions. None of them saw him as a victim.
I cannot think of a similar protest, or condemnation, having happened in Northern Ireland when the IRA murdered a police officer and civilians.
In Manchester, the Muslim community was again quick to condemn the atrocity. Their message was clear; it said the people who perpetrated this act are not representative of Islam, but are extremists who disgrace the religion. The more Imams and Muslim leaders that spread this message the better.
The pain and misery of thousands of innocent victims and survivors of terrorism in Northern Ireland would be mightily relieved by words of this kind from Sinn Fein.
Ultimately, the point being made by this article is that, the unarmed British bobby style of policing is investigative-led. The police rely on the consent of the community. It is a not a counter-terrorism model. It is not a natural fit for counter-terrorism intelligence. Regional counter-terrorism police hubs under a variety of names are an effort to offset the shortcoming. But the set-up is fine.
Violent extremists crave repressive security measures to attract sympathy. The last thing they want is a bobby on the beat talking with local people. I have worked on overseas projects with superb officers from Greater Manchester Police. They get it. Manchester is in good hands.
The greatest weapon the police have against warped men like Abedi is the Muslim community. This is because the police depend on information from the local community to catch offenders and prevent future attacks. MI5 plug into this large operational footprint.
Understanding this, as MI5 and Greater Manchester Police do, is why this evil that offers nothing but death, destruction and division will be defeated.
Combating terrorism is not about containment. It is about beating a bad ideology with a good ideology.
With the challenges MI5 face in mind, a re-elected government may look to unearth the intelligence lessons of the Troubles that a "peace process" has buried.