The photograph is like countless others released by Islamic State's propaganda wing: a young man sits flanked by a pair of Kalashnikovs, grinning excitedly at the prospect of proving himself in combat.

Yet when Seifeddine Rezgui's turn for martyrdom came, it was not in the tough battlegrounds of Syria or Iraq, but in a killing field that could scarcely be easier - a group of unsuspecting tourists sunbathing on a Tunisian beach, some of whom may not even have seen his face.

According to Tunisia's prime minister, Habib Essid, Rezgui was a 23-year-old engineering student who originally hailed from Gaafour, a drab and unremarkable town some 80 kilometres south-west of Tunis. He had never travelled abroad, Mr Essid said, and was not known by the police to hold extremist views.

"We were all shocked when we heard the news and saw his picture, and his mother was devastated," said his uncle, Ali Al-Rezgui. "Yes, he went to mosque and prayed, but just like the other young men, he would then go to a cafe and play football with his friends.

Another suspected militant Islamist who was probably acting alone has been accused of decapitating his boss and attempting to blow up a gas plant in southeastern France. Yassim Salhi, a 35-year-old of North African parentage, is being held in the city of Lyon after ramming his delivery van into a warehouse storing gas containers on Friday. He was apprehended minutes later while opening canisters containing flammable chemicals. Salhi, whose wife and sister are also being detained along with a fourth person, is known to have associated with Islamists over more than a decade and had previously been flagged as a potential risk.

Investigators searching his home in Saint-Priest, near Lyon, seized a laptop, a tablet computer and a fake pistol but found no trace of explosives or radical propaganda. "We don't know whether we're dealing with a fundamentalist who just lost it or a real terrorist," a source said.

Salhi took a macabre "selfie" with the severed head before his arrest, a source close to the investigation said on Saturday, and then reportedly sent the photo to a Canadian mobile phone number via WhatsApp.

Rezgui, too, was adept at internet technology: his Facebook page  includes fan postings about rap music, Real Madrid and Tunisia's chances in the Africa Cup of Nations, along with numerous postings in support of Islamic State, as well as several of its propaganda videos.

Notably, most of Rezgui's radical postings appear to have started only early last year, prior to which he seemed to be more interested in rap music and sharing jokes. His final entry is dated New Year 2015 and reads: "May God take me out of this unjust world and perish its people and make them suffer. They just remember you when they die."

Why Rezgui stopped all his Facebook activity after that date will now be a matter for the authorities, which on Friday seized a laptop during a raid on his family home in Gaafour, where his parents were also arrested. But a clue may lie in Rezgui's Islamic State nom de guerre, Abu Yahya al Qayrawani. The last name is a reference to the Tunisian city of Kairouan, where Rezgui last year enrolled for a master's degree at the local university.

The ancient city, which sits on the edge of the Sahara desert, was one of the earliest centres of Sunni Muslim Islamic scholarship and has long been considered one of holiest sites in the Muslim world.

More recently, though, it has also been a centre for Islamic radicalism, and until two years ago was the main gathering place in Tunisia for Ansar al-Sharia, the radical Islamic group that first sprang up in neighbouring Libya after the 2011 Arab Spring.

Following Ansar al-Sharia's attack on a US embassy compound in Libya in 2012, in which ambassador Chris Stevens was killed, it was declared a terrorist group by Washington. At the time, the Tunisian government chose not to follow suit, wary of being seen to act in the same authoritarian fashion as the country's deposed dictator, Zine El Abidine Ben Ali. But in 2013, it too outlawed the group, after a riot erupted when police stopped its followers holding their annual rally in Kairouan.

Since then, many of its hardcore members are believed to fled abroad to fight with Islamic State in Syria and Iraq. Such is the exodus that Tunisia is now believed to have contributed some 3000 jihadist fighters to the conflict, more than any other country.

"If jihad is a crime," Seifeddine Rezgui wrote in his last Facebook posting, "the world shall know that I'm a criminal."

Telegraph, London, Reuters