The door is an angry shade of purple, like every other door in the seven-storey block. It has no handle, and no name beside the buzzer – just a heavy lock, and spyhole for checking out unwelcome callers.

The building is a vast and seemingly impenetrable concrete warren divided into some 600 apartments – or perhaps ‘boxes’ would be a better term, given their cramped proportions.

Surrounded by dozens of similarly forbidding, state-owned monstrosities housing some 10,000 Muslims, the vast majority of North African origin, this is the grim Parisian enclave where Cherif Kouachi was radicalised – and is believed to have plotted the horrific magazine massacre.

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A history of violence: Youths set cars ablaze and roam the streets as they battled police in Paris in 2005

From the Marais, the chic quarter where Charlie Hebdo’s offices are situated, it took my taxi barely 20 minutes to reach his seventh-floor flat in the northern suburb of Gennevilliers yesterday. Though it is just seven miles from the scene of the carnage, once you pass beneath the thundering Peripherique ring-road and enter this soulless district, you have reached a parallel universe.

Half a century ago, Gennevilliers was an archetypically French community, as illustrated by the charming sepia photographs hanging in the Brazza brasserie, opposite the town hall, showing people dining on shady terraces, and riding bicycles in their berets.

Then, the faces were almost all Gallic. Today, you can walk the length of a crowded boulevard here without seeing a solitary white Frenchman, and the idyllic ambience lives only in the memory of the very few older residents who remain.

It has been replaced by a menacing edginess, and – when approached by a British reporter seeking to understand why young French-Arabs are turning to Islamic extremism, apparently in ever increasing numbers – a deeply ingrained sense of mistrust.

A decade ago, after the incendiary Parisian race riots of 2005, I ventured to another of these seething ‘banlieues’, to be greeted with extraordinary courtesy by youths who had, a few hours earlier, been hurling petrol bombs.

Then, they were eager to air their deep sense of disaffection. Yesterday, in the Arab café and kebab shop Cherif frequented, I met with shrugs of indifference and hostile stares.

It was left to Mohammed Benali, president of his local mosque where 3,000 people worship each Friday, to explain why, for ‘a tiny minority’ of young French Muslims, violence was their chosen path.

A car burns during clashes almost a decade ago in the Parisian suburb of Aulnay sur Bois during riots

During the Sixties and Seventies, he said, when immigrant labourers from Morocco, Algeria and Senegal were shipped in to rebuild post-war France, suburbs such as Gennevilliers (where they worked mainly in car factories and heavy industrial plants) were utterly transformed.

Yet for this first generation – well-paid and comparatively well accommodated in the vast new tower blocks built to house them – life was satisfactory enough. However, it was not the French policy even to attempt to integrate or educate them. On the contrary, they were pushed to the margins, to be tolerated rather than welcomed, and among the indigenous French population the level of discrimination was beyond anything we have witnessed in Britain.

The problem manifested itself in the second generation – to which Cherif, an adopted, French-Algerian orphan born in the mid-Eighties, belonged.

‘They saw how their parents were treated and felt deeply resentful,’ M. Benali told me. ‘After the 2005 riots I was among a Muslim delegation who met Nicolas Sarkozy (then interior minister) and I’ll never forget his words.

The seven-storey block is where suspect Cherif Kouachi (pictured) was radicalised - and is believed to have plotted the horrific magazine massacre

‘He told me that “to humiliate is to radicalise” – and that is exactly what came to pass. This second generation felt excluded, discriminated against, and most of all, humiliated. Added to which, they were deeply mixed up – they spoke and felt French, but were regarded as Arabic; they were culturally confused.’

Among the elders at his mosque, he said there had been lengthy discussions about the offensive Charlie Hebdo cartoons, and whenever they were published feelings ran high. Yet, he maintained, worshippers – particularly the younger ones – were encouraged to accept the principle of free speech, and embrace French values along with their Islamic beliefs.

But Cherif – an occasional worshipper at the mosque – was among a handful who refused to follow this instruction, he said.

‘When the imam told everyone to enrol on the register of electors so they could take part in elections, and play their part in society, he refused. He said he wasn’t a French citizen and wanted nothing to do with the democratic process. He then walked out of the mosque. But these beliefs didn’t come from the imams who teach here. He had been who knows where. He was using the internet. He was crazy; brainwashed. He was beyond our reach.’

Perhaps so, yet according to one friend, a surly young man who lived in his block who agreed to speak anonymously yesterday, Cherif was an unlikely killer. The sort who would walk up five flights of stairs rather than share the tiny, often uncomfortably crowded lift in his apartment block.

However, to Eric Badday, a 60-year-old Tunisian-born writer who has lived next door to him for seven years – and saw Cherif’s young wife being led away in handcuffs on Wednesday afternoon – it came as no surprise that he is being hunted.

‘Cherif wore Western clothes, unlike his wife who dressed traditionally, but he was the type who believes Allah is the cure for all life’s ills, from social problems to depression,’ he told me. ‘You could just tell he was a fanatic.’ A

ccording to Professor Olivier Roy, a French expert on political Islam, this impression fits perfectly with the stereotype of a frightening new breed of alienated young French-Arabs living in the shadowy margins of Gallic society. Young men who have been ‘radicalised through de-culturisation’, as he puts it.

A vast block in Gennevilliers, which is a seemingly impenetrable concrete warren divided into some 600 apartments

‘These are second or third generation descendants of immigrants,’ he told the Daily Mail. ‘They don’t speak Arabic; they don’t wear traditional Arab clothes or eat traditional Arab food or listen to traditional Arab music. They do not speak the language of their parents and have not been educated in religion by them.

‘They have the sense that they are living in a disenfranchised ghetto and then there is a process of self-radicalisation... it is a form of Islam totally opposed to any culture, Eastern or Western, and has its own set of explicit order – do this, don’t do that.

PAIN OF EDITOR'S PARTNER The partner of Charlie Hebdo editor Stephane Charbonnier wept last night as she paid tribute to her ‘hero’. Jeannette Bougrab, 41, said: ‘There will be a before and an after but I have lost Stephane.’ The lawyer and ex-government minister hailed the cartoonist, nicknamed Charb, and his colleagues, saying: ‘They were heroes. I was with a hero that I loved.’ She added: ‘He was executed because he was defending secularism ... they fought for principles and freedoms that we forgot to defend. ‘They died so that we can remain free in this country.’ Advertisement

‘These young people are out of reach of the traditional Muslim authorities. They don’t care what the imams say or the leader of the Grand Mosque in Paris. They radicalise between themselves.’

For years, it seems, Cherif maintained a low profile, living on casual work and, like many young Muslims (who claim with some justification to be last in the queue for jobs), on welfare handouts.

His disenchantment hardened into radicalism, and an 18-month spell in jail. Given that 80 per cent of French prisoners are Muslims, yet no attempt is made to de-radicalise the fundamentalists among them, he emerged festering with even more anger.

Nothing can fully explain the depths of hatred and cold-blooded barbarism that motivated those three young Frenchmen on Wednesday morning, of course.