One strange consequence of my job as a foreign correspondent is discovering beautiful places when terrible things happen in them. So it was that I have been spending the past couple of weeks in Tunisia, a land of azure skies, whitewashed houses and apricot light which has inspired artists such as Paul Klee. That beauty — along with soft sandy beaches, local rosé and low prices — also attracted hundreds of thousands of British tourists. Not any more, after a young Tunisian took a gun from inside a beach umbrella at the resort of Sousse and slaughtered 38 holidaymakers, 30 of them British. Almost every Tunisian I met apologised on learning I was British: ‘Please don’t think Tunisians are like that — we are people of peace.’ But as one visitor who stayed on after the attack, a beauty consultant at Selfridges, said to me, ‘It’s hard to look at the beach in the same way after this.’

I had long wanted to go to Tunisia to try to understand a conundrum. This North African country is where the Arab Spring started, when a fruit-seller set himself alight after police confiscated his wares. It’s often cited as the movement’s only success story, having toppled the dictator Ben Ali in 2011 and replaced him with an elected government and constitution. Yet Tunisia sends the most fighters to Isis — an estimated 4,000 currently in Syria, not to mention others in neighbouring Libya. To find out more, I went to the Interior Ministry. At the gate, police were beating a man, which wasn’t a good sign. Inside I met Waleed Lougini, a lugubrious spokesman who like most Tunisians smokes non-stop. ‘People keep saying we have the most fighters in Isis but we are only fourth or fifth on the list,’ he said. The ministry claims to have blocked 12,000 from going and recently brought in a law requiring anyone under the age of 35 travelling to transit countries like Turkey to have permission from their parents. That only seems to have annoyed people.

Tunis by night is incredibly lively. I try to avoid going to Muslim countries during the month-long Ramadan fast, because everyone is moody and sleepy in daytime. But my goodness, in Tunis they come alive after Iftar, the fast-breaking dinner. At about 9 p.m. everyone pours into the streets and sits in cafés or ice-cream parlours, drinking coffee, smoking shisha pipes and people-watching. One of the most popular places is the tree-lined Avenue Habib Bourguiba — or Rue de Revolution as many now call it. In all my travels in the Islamic world I have never seen such a mix — girls in full hijab walking with girls in spaghetti-strapped mini-dresses or tiny shorts. It’s easy to be fooled into thinking this is a country that has it right. But not long ago Tunisia was a brutal police state and I saw reminders every day with gratuitous displays of violence such as that at the ministry gate. Opposite the cathedral on Avenue Bourguiba is the French embassy. Surrounded by tanks and barbed wire, it does not inspire confidence.

Imagine being the parent of the cold-blooded beach killer. On my last but one day I headed into the interior, a soft landscape of rolling hills dotted with olive trees and storks standing in nests on poles, to the town of Gaâfour. Inside a whitewashed compound on the corner of a street sat a woman who looked a portrait of melancholy — Radhia Manai, the mother of Seifeddine Rezgui. ‘My son was a normal boy,’ she said over and over, wiping away tears with her grey shawl. She had not been able to eat or sleep since the moment police told her that not only her beloved son was dead but that he had massacred 38 tourists. She showed school reports and photographs of a harmless-looking young man sporting different hairstyles. ‘He liked new looks,’ she said. Knowing many young Tunisians are joining Isis, she had warned him to stay away from salafists at his university. ’Don’t worry, Mum, I am not one of them,’ he assured her. At the local youth centre, friends who were with him talking football just two nights before the killing were stunned. ‘What this means is these people are among us,’ said one with whom he did breakdancing. The scary thing about Rezgui is that he seems to have given away no clue to either friends or family — except for telling his father on New Year’s Day that he wanted to move university.

As the interior ministry man had said, ‘This is a problem for all the world not just Tunisia.’ On my first day back in London I went to a Foreign Office panel on extremism — how to win over those who are being radicalised. What Rezgui showed is we don’t even know who they are.