HE STRANGLED his employer in the back of a van, hacked off his head with a kitchen knife and attached it to the gates of the chemicals factory where he worked, next to two Islamic banners. Yassin Salhi, a French citizen, confessed this week to a grisly murder on June 26th, at Saint-Quentin-Fallavier near Lyon, that shook France. Four days later, the public prosecutor concluded that Mr Salhi had acted with a “terrorist motive”.

The French have become grimly accustomed to news of beheadings by jihadists in far-flung places. Last year a French mountain guide was decapitated in Algeria by a group linked to Islamic State (IS). But such a horror on French soil was a far greater shock. After displaying the severed head, Mr Salhi rammed his van, loaded with gas canisters, into the chemical plant, in what appeared to be a failed attempt at a suicide attack.

Although no group has claimed responsibility, the public prosecutor said that the aim was clearly to terrorise with “maximum publicity”. Mr Salhi sent two photos of the severed head, one of them a selfie, to a French convert in Syria, and cried “Allahu akbar” (God is great) as he tried to set off further explosions. His sister told investigators that he had spent a year in Syria in 2009 (before IS was formed). Thanks to contacts with radicals, from 2006 to 2008 Mr Salhi was on an intelligence watch-list.

The attack has reminded the French of their vulnerability. Since France joined air strikes on IS in Iraq last year, several jihadist videos have called on the faithful to strike French infidels. “If you don’t manage to get a gun, there are stones and knives,” urged one. Home to Europe’s biggest Muslim minority, France has been a fertile recruiting ground for “self-service” jihadists, such as those who committed the murders at Charlie Hebdo and in a kosher supermarket in January. “France has the sad honour of supplying the most numerous contingent” of Europeans to fight in Syria and Iraq, noted a recent Senate report. French citizens account for nearly half of the 3,000 or so Europeans known to be in the two countries or on their way there. At least one French citizen is suspected of taking part in IS executions; seven French citizens or residents have carried out suicide attacks.

The use of a kitchen knife demonstrated that arms control and surveillance are only part of the battle. Leaky borders and a continental location mean that there are far more illegal guns circulating in France than in Britain, says one former foreign intelligence official. Yet Mr Salhi had with him only a fake gun. He decapitated his boss with a 20cm (8-inch) blade, the sort used domestically to slice tomatoes.

So the focus is on identifying and stopping those drawn to jihadist terror. François Hollande, the president, has already boosted the intelligence budget, legalised various forms of electronic eavesdropping and begun to recruit more spies. But it will take time to turn trainees into enhanced operational capacity. As it is, some 3,000 French individuals are thought to be “involved in terrorist movements”, according to Manuel Valls, the Socialist prime minister. The intelligence services cannot monitor them all.

The government has now upped the political rhetoric. Islamist terror represented a “war of civilisations” declared Mr Valls, to the consternation of some on the left who regard such martial terminology as belonging to the right. A former interior minister, Mr Valls has made tough talk his hallmark. After the January murders he told high-school students that they had “to learn to live with the danger” of terrorism. Such words shock, but after this latest attack they do not appear excessive.