British-born, known to the intelligence services and the police, previously questioned about his extremist views and holding convictions for assaults, including GBH, possession of offensive weapons and public order offences. Could Khalid Masood have been stopped?

It is an inevitable question in such tragic circumstances, and a fair one at least to pose. In every outrage the security forces, their political masters and the wider population has to be satisfied that every possible practical precaution had been taken. There may have been mistakes in this case, or there may not – none of that is so clear now. What we do know, however, is that only in a totalitarian police state is it possible to eliminate all possible threats to security, and in fact not even entirely reliably in such circumstances.

The British do not wish to live in a police state as the price of improved security; absurd as it is, that would only represent a victory for the terrorists themselves who wish to destroy our way of life.

Much the same goes for ever more draconian measures to invade privacy and erode civil liberties. Almost mundane questions will need to be asked about the quality of intelligence and surveillance over this individual and any associates or accomplices. But it is too premature to condemn the authorities for incompetence. Besides, we should take them at their word that they quietly thwart scores of planned deadly attacks every year; the record, so far as can be judged, is not one to be ashamed of.

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

Nor is it sensible to believe that “extremism” can ever be eradicated. The Prevent strategy has many critics, inside and outside Muslim communities, and could no doubt be reformed to make it more sensitive and more effective. Hate preachers are all too common, but there are already laws that can control them, and the pressure of the wider Muslim community to minimise their influence on impressionable young minds.

It is also an uncomfortable truth that the vast library of jihadi material available freely on the internet is near-impossible to regulate. Multiculturalism is hardly to blame for that. If it has been as profound a failure as Nigel Farage and others claim, there would be far more incidents than the ones we have witnessed. Nor can it be blamed for the murderous assault on Jo Cox MP last year.

The backlash that the enemies of democracy on the extreme fringes of British society yearn for has not come. People generally recognise that their Muslim friends, neighbours and work colleagues wish nothing more than the opportunity to make a better life for themselves and their families, as does everyone. Their leaders have vocally condemned the violence, though none of it is their fault, and the daily contributions millions of our fellow Muslim citizens continues.

With previous generations of terrorists – Irish republicans, Basque separatists or the far left – there were identifiable individuals, though vile, who could be negotiated with. The funeral of one such individual, the reformed IRA gunman Martin McGuinness, was held on Thursday, a reminder of how so many conflicts involving the British have ended up with terrorist leaders becoming leaders of their independent nations and taking tea with the Queen.

Even if it were sensible to engage with Islamist organisations such as Isis, they have little, if any, control over those “inspired” by the myriad perversions of Islam circulating on the web. Sadiq Khan, who as London Mayor has shown leadership and statesmanship sadly lacking elsewhere, has been much criticised, and much misrepresented, for holding the view that terrorism has become a fact of modern life in big cities – but he is right.

Islamist terrorists – not “Islamic”, as Theresa Amy also correctly stressed – and the deranged individuals who take it on themselves to act in their name prefer to attack high-profile world-famous landmarks in media hubs such as Berlin, Paris and London, or New York and Washington as on 9/11.