There was no warning. Geraldine learned her son was dead from a text message sent by an Islamic State (Isis) commander. “Congratulations,” it read. “Be proud of him. He is now a martyr. Be happy he died fighting the unbelievers.”

Then came another message, this time from a young man who, like Anis, had travelled from Belgium to fight in Syria and had ended up with the extremist group.

Anis had died in combat, it told the bereaved mother, killed cleanly by a bullet between the eyes. “He had been buried on the field of battle and was missed by all his friends.”

The messages arrived on her phone 10 months ago. Geraldine, who has asked that her family name be withheld, still has no proof of her son’s death, nor precise knowledge of where his remains lie.

“It was destiny that he died,” she said, weeping.

Anis, who was 19, had left his home in Molenbeek, a poor neighbourhood in west Brussels, a year before he was killed.

For months now, Molenbeek has been under intense scrutiny. Investigators believe its brick-house-lined lanes were the operational base for the network that launched terrorist attacks in Paris in November, which left 130 people dead. At least three of the 12 gunmen who struck the French capital, and more who were involved in the support network, were from Molenbeek. At least half may have been veterans of the war in Syria.

On Tuesday, police investigating the Paris attacks raided an apartment in the Brussels suburb of Forest. It followed a number of raids in Molenbeek in February “carried out as part of an investigation into a recruitment network linked to [Isis]”, an official statement said.

Anis had travelled from Molenbeek with a close friend and the younger brother of Abdelhamid Abaaoud, the suspected mastermind behind the Paris attacks. All three are thought by counter-terrorist officials to have joined the Belgian and French “brigade” within Isis, though his mother doubts this. She says he was in charge of logistics, but does not know for which group.

“People call me the mother of a terrorist. I am not. They talk about radicalisation, but they don’t really understand what that means,” said Geraldine, a 50-year-old financial administrator who converted to Islam 25 years ago and married a man of Moroccan origin.

Her son, fluent in Flemish and French as well as classical and colloquial Arabic, had attended a local school. He had not been the best student – “a bit of a brawler, easily led, but popular”, according to his mother – but had decent basic qualifications and was a keen sportsman.

Muslims pray at the Al Khalil mosque in Molenbeek. Photograph: Francois Walschaerts/AP

The family were not poor. But then nor were those of the three Paris attackers raised in Molenbeek. The Abdelsalaam family – one brother blew himself up on the rue de Voltaire, while another is on the run – lost social housing benefits when their declared revenue passed €100,000 three years ago, according to Le Monde newspaper.

And if Geraldine and her husband were practising Muslims, who prayed five times a day and fasted, Anis was less observant. “He had always been laid-back about that kind of thing,” she said.

This, too, appears to have been the case with the Paris attackers, who were reportedly drinking alcohol and smoking cannabis until relatively recently.

Anis was radicalised over just a few months. “He had left school and been trying to get jobs. It was difficult. It’s tough for all the youngsters but if you’ve an immigrant background doubly so. He started to say that people saw him as Moroccan while in Morocco they saw him as Belgian and asked me ’who am I?’” Geraldine said.

Within weeks, Anis started talking about Palestine and Israel, then Syria and then about Islam, saying that it was the duty of all Muslims to help those in distress.

“He was arguing with my husband, sometimes violently, about the meaning of the Qur’an. He said it was obligatory for us to go to Syria, and if we wouldn’t, then he would,” Geraldine said.

Belgian security forces during an manhunt operation in Molenbeek last month for two suspected terrorists linked to the Paris attacks. Photograph: Olivier Hoslet/EPA

When his parents realised they could not dissuade him from travelling, they told the police “anti-radicalisation cell” in Molenbeek, believing the authorities would stop their son leaving. A week later, Anis called his mother. He was in Turkey, he told her, and would be in Syria within hours. A magistrate, she later learned, had decided that as Anis was an adult, it was not possible to impose a travel ban.

The friend who left with Anis was killed shortly after their arrival in Syria. “I was speaking to him every week more or less. When his friend died, I heard doubt in his voice for the first time. But not for long,” said Geraldine.

Reporting in recent months has focused on the concentration of violent militants in Molenbeek. But, according to experts, the problem may not be about places, but people. Montasser AlDe’emeh, a 27-year-old researcher who lives in Molenbeek and counsels former and current fighters, said

he personally knew one of two people who fought in Syria who were killed in January in a shootout with police in the town of Verviers.

“He used to come into the cafe where I go from time to time. Everyone knows everybody round here. They talk, share videos, make plans. That’s how it works ,” said AlDe’emeh.

Research from Oxford University confirms the importance of social networks, showing friends or peers played the primary role in the recruitment of three-quarters of foreign fighters to Isis. Family members accounted for a fifth, mosques for just one in 20.

The role of mosques is controversial. AlDe’emeh said he knew of several clerics preaching in mosques who had recently travelled to Syria. “Imagine what they were telling their congregations,” he said.

But Abdelilla, a social worker with 20 years’ experience in Mollenbeek said mosques, whether officially registered or not, were not a major problem. And Geraldine said her son never attended mosques but had got involved with people “on the street”.

Like many parents, she initially welcomed the new religious fervour of Anis. “I thought it would give him a direction,” she said.

Her son had no criminal record. But a very high proportion of the Paris attackers did. AlDe’emeh believes there are two profiles of militants: the naive idealists who are the “first wave” to travel to Syria and a “second wave” of much more violent extremists prepared to strike in their homeland. The latter have often long histories of involvement in sometimes serious crime.

Geraldine believes her son fell in the first category. “He was happy there. He was living his ideal. He did not leave wanting to fight a war but to live as a good Muslim,” Geraldine said.

But in the narrow and brutalising confines of a conflict zone, another man emerged.

In weekly conversations Geraldine encountered “two different Anis”. One was “his mother’s son”, the other a hardline fanatic, who stopped talking to his father on the telephone after repeatedly telling him he was a coward and a “bad Muslim” for living among unbelievers.

“Sometimes we just talked about home, about what was happening, about his sister. Others times – when I think other people could hear him – he was very harsh. Once he asked me if I would buy a ticket for him from Turkey to a third country, so he could leave Syria without coming back to Belgium. I said, of course. But hours later he told me to forget it.”

Anis was killed during an attack near the strategically important airport at Deir Ezzor, a town in eastern Syria held by Isis.

Belgian authorities have recently announced a £200m plan to counter extremism. It involves hundreds more security officials, an expansion of surveillance cameras and intensified checks on public transport.

Geraldine believes much could be done to prevent young men leaving Belgium to fight in Syria in future.

“Muslim intellectuals need to make a much stronger stand and speak out much more,” she said. “And when a mother tells the police her son is leaving for Syria and that she wants them to stop him, then they should.”

In recent months, with increased airstrikes and intense combat, casualties have mounted among the foreign Isis fighters. “Every week we hear of another Belgian who has died. I am almost glad my son is not alive to see it,” Geraldine said.