They gathered, one week on to the minute from the assault of Friday the 13th, around what seemed to be a shadow devoid of life and light – a heavy black tarpaulin draped over the entrance to the Bataclan theatre: or “ba’ta’clan café”, as the awning reads. The previously illuminated sign is still there, now unlit: Nous Productions Présente Eagles of Death Metal.

Across the road: the vast blanket of candles, flowers and personalised tributes. They form a lineage, somehow, from Washington Square on 11 September 2001, then Place de la République up the road on the night of 7 January this year, now this. Acts of intimacy and frail hope in a vortex out there, of mad politics and distant wars come vengefully home to claim those who had simply gone about their lives. In New York, they had been mid-ranking traders; in January cartoonists and journalists; now it was the turn of young folk out for a drink or pizza – and rock fans.

The hour struck, 9.17pm, and leaden silence fell, broken only by the babble of TV reporters beside their mobile trucks. Flags affixed to the fence: Sardinia, Lebanon, Puerto Rico, Brazil. Teddy bears, a guitar with le Bataclan est toujours vivant – the Bataclan lives forever. A paperback of Hemingway’s A Moveable Feast, translated as Paris est une fête – Paris is a party. A Paris Saint-Germain banner with an epic poem written on it and special joint plaque made by the supporters’ clubs in France of Liverpool and Everton. And a black vinyl LP, chipped and cracked, with a white flower in the middle – and attached, the darkly bitter words: Rock Kills You.

The story had been told incrementally over the week: by people such as theatre assistant Jean-Pierre Bouyxou recalling the reloading of magazines and shooting those who pleaded, urging his wife: “Play dead and above all keep down. We can survive this.”

The bloodletting had names: two gunmen who came here to execute these “hundreds of idolatrous sinners” attending a “festival of perversion”, as Isis repulsively brands young fans of rock’n’roll. One was called Omar Ismail Mostefai, the other Samy Amimour.

The story of these two men illustrates with cruel clarity the choices that France faced last week – and faces into its future – between two of the republic’s cherished traditions. One is the principle, and figure, of liberté, first and foremost in the secular trinity proclaimed by the revolution. Personified as Marianne, she was ubiquitous last week: one cartoon in a magazine of that name adapted Delacroix’s great painting of her leading the people to show liberty clad in blood-drenched robes leading hi-vis-jacketed paramedics and survivors wrapped in thermal blankets over ground strewn with bodies.

The other is the tradition of the strong state ostensibly representative of the people, an evocation of the name of one of the republic’s first institutions, established in 1791, the Comité de sûreté générale – committee for general security. A few hours before the carnage of Friday the 13th, a general called Jean-Louis Desvignes, president of the Association of Reservists, Arcsi, posted a document on an internet forum debating “digital freedom”. It was headed by the words: “Liberté, fraternité, surveillance”.

A bitterly prescient joke, for it is in the interests of these other notions of security and surveillance that dark clouds now gather over liberté, herself apparently willing to draw curtains over the very freedoms she eschews. “The questions of the past,” the post read, “are always with us in the present. How do we find a balance between liberty and security?” Two days and 129 lost lives later, a headline in the newspaper L’Opinion talked about a new “Maginot Line of internal security” – invoking another line through the heart of France: between liberty versus security.

The polls became progressively unequivocal. An initial test of the water by iTélé showed a majority in favour of “measures to restrict liberty of individuals to preserve the liberty of all”, but respondents were less inclined to declare that: “I am ready to see my liberty reduced if this benefits the general interest.” Two days later, a poll by Ifop and Le Figaro showed 84% ready to accept “controls and a certain limitation of liberty in order to guarantee better security”. (Another poll, by CSA, put the figure lower, at 63%.)

According to Ifop, even voters for the far-left Front de Gauche were 65% in favour; for Socialist voters it was 87% and there were inevitably overwhelming majorities for the centre-right and National Front. Ifop’s director, Jérôme Fourquet, surmised that “the French do not only adhere to the rhetoric of ‘war’ – [prime minister] Manuel Valls talked about ‘war’ last January – but also to decisions that entail a restriction of public liberty”.

The centrepiece of the entrenchment of state power is a proposed change to the constitution, establishing recourse to a “civil state of emergency” in addition to powers adopted in 1955 during the Algerian war of independence, in order – says François Hollande – to confront “war of another type”, as presented by jihadi terrorism.

There is little opposition, apart from a caution in an editorial by Le Monde, that “to make the state of emergency a permanent arm of executive power could lead to a jeopardy of fundamental liberties. This places security at the first level of the rights of man. Security is an expectation, but not at any price.”

Proposals for the stripping of nationality from French-born dual nationals engaged in jihad would be controversial under any other circumstances, and Florian Borg, president of the Union of Lawyers, was almost a lone public voice protesting that this might “accentuate the stigmatisation of certain French [citizens]; moreover, the stripping of nationality can also be counterproductive”.

The police will be reinforced by 5,000 officers, other services by a further 5,000 and cuts to the armed forces are frozen until 2019. The military can shed its so-called peau de chagrin, skin of resentment, as soldiers take their place across Paris suburbs that had already come to feel like a casbah under siege, and in the lives of the inner city, now with broad public consent.

A further facet of last week’s measures was to continue the state of emergency for three months, so that posters all over Paris calling for a “march for climate” to greet the environmental summit opening on 30 November cry into the ether; regional elections will take place without campaign rallies.

There was one lone and loud voice against another pillar of the presidential and national reaction, to intensify bombing Isis. This was the man who was to the fore at the genesis of all this, when the Bush and Blair administrations prepared to invade Iraq during the autumn of 2002: the then French foreign minister Dominique de Villepin.

A year ago, De Villepin, now retired from electoral politics, went on television to argue that “the war against terrorism cannot be won. The failure is already apparent. They are opportunists, a hidden hand.” He warned France against “following the United States in ideological messianism” and of “hatred against hatred, war against war”, urging “a political vision, political strategies”.

Last week, De Villepin said: “I don’t take it back. I don’t want to play the terrorists’ game. To make us think we are at war is a trap. They’re not an army of state, they’re groups of fanatics.” By bombing them, “we legitimise their claim to be at war. Indeed, they want to destroy us … they want to divide us and push us into civil war. But because a band of murderers and fanatics declare war, we shouldn’t fall into the trap of outbidding them”.

There is no serious consideration of a government of national unity in pursuit of Hollande’s strategy, but his stance puts the rightwing opposition on the back foot, with little room for manoeuvre but to support him – and advance more stringent measures. This has begun, cutting to the core of the question: how did the killers mount such an operation undetected, and how to prevent another? This is a discourse that invokes a little-discussed kernel of France’s efforts to trace its assailants: the so-called S Files.

S Files are kept on individuals monitored for potential threat to security, kept by the Direction Générale de la Sécurité Intérieure – descendant of the committee of 1791. They have counter-espionage origins in the cold war, and include militants of the far left and right, and soccer hooligans. But the DGSI is now reported to hold at least 5,000 such files, three-quarters of which relate to Islamist extremism.

To qualify, individuals need not have committed any crime, but must belong to an organisation deemed a “menace to the public peace”, such as an extremist mosque. You cannot be arrested for having an S File – it is “an alert”.

S Files overlap with other lists, including those in a special investigative folder established after the attack on a gas facility in Isère last June, dedicated to “the assembled spectre of radicalised persons”. Names are sub-categorised according to threat and type: S14 defines those known to be in, or returned from, Syria or Iraq.

Omar Ismail Mostefai opened fire on the rock fans as a name on the S Files, registered in 2010 as an ultra-orthodox activist based in Chartres. On arrival there, he made contact with an imam and after two years vanished, only to reappear in autumn 2013, entering Turkey with two men – among them Samy Amimour, the other killer at Bataclan.

Amimour was also known to DGSI, qualifying for an S File in 2012 after an aborted attempt to fight in Yemen, and placed under “judicial control”, obliging him to report to police – a control he slipped by travelling to Syria, thereby proceeding to an international arrest warrant.

Others with S Files include Mohamed Merah, who killed seven people, including Jews and soldiers, in Toulouse and Montauban in 2012; Mehdi Nemmouche, who attacked the Jewish museum in Brussels last year; Chérif and Saïd Kouachi, the Charlie Hebdo killers; Amedy Coulibaly, one of the killers in a Paris kosher market on 9 January; Sid Ahmed Ghlam, implicated in a murderous attack on a church in April; Yassin Salhi, arrested for the gas plant attack in Isère; and Ayoub El-Khazzani, who recently tried to murder passengers aboard a train from Amsterdam to Paris. All on the S Files.

The fact is – whether state services and civil libertarians like it nor not – the S File system has identified many of those involved in mass murder. The question is: what to do about it?

Marc Trévidic, a former anti-terrorist examining judge for Paris, has for some time been warning of those on the S Files. In September, he told Paris Match that Isis has “the ambitions and the means to hit us much harder [than last January], organising bigger actions than we’ve seen so far”.

Now Trévidic – speaking from Lille where he has moved due to limits on terms of service – says: “The services knew such an action was imminent. But how can we know where the act of terrorism will happen? All those on the lists represent a potential menace, but what can we say about those who aren’t listed, who are no doubt numerous? We’ve managed to give a face, in our procedures, to people who’ve made journeys to Syria. There are some who’ve made the journey down there without figuring on our lists. But certain individuals have managed, despite the surveillance. The sheer number of people listed is itself a problem.”

As last week progressed, one proposal came from the wings to centre stage. It had been suggested in September by Arno Klarsfeld, son of the Nazi-hunter Serge Klarsfeld. He proposed on a website called Causeur that those identified on the S Files be put into “administrative detention”. “Do we have to wait for the next catastrophe, or can we try to prevent it?” asked Klarsfeld, citing passages in the European convention “in time of war”, and prime minister Valls’ invocation of “war” before Friday the 13th.

Ten days ago, the idea of some Guantánamo Bay arrangement in the heart of Europe would have been considered preposterous, and Klarsfeld’s proposal vanished. But last week, Laurent Wauquiez, number three in Nicolas Sarkozy’s opposition Republicans, resurrected the idea. “Behind the attacks,” he said, “there are always people already identified by the authorities in the S Files, known to our services for their radicalisation but not arrested on the pretext that French law does not permit it. I propose precisely that we change our laws so as to permit us to act before these things happen, and place them in internment centres where they cannot harm us. I think it’s a sensible proposition which the French people will understand.”

The S list is not supposed to be limited to the services of one country, but shared across the Schengen zone. Trévidic talks about “the porosity of the Franco-Belgian border” so far as cells operating in both countries are concerned – the axis between Paris and Brussels that has driven the wave of attacks since Charlie Hebdo.

The Brussels suburb of Molenbeek used to be known as Little Manchester, such was its prowess as a manufacturing hub – no longer. Yet it is here, back in his home town, that investigations focus again after the killing of Belgian Abdelhamid Abaaoud in Saint-Denis on Wednesday; here that his alleged co-conspirators may still be hiding, where the Paris plot was hatched. The foggy air is thick with distrust, a state of siege – and insinuations from France that the Belgian authorities failed to communicate all they knew.

In Brussels last week, an official with the prosecutor’s office told the Observer he feared that “some of the information gathered for the equivalent of the S Files in Belgium was not shared with the French authorities”.

A Belgian file, detailed in Saturday’s Le Monde, traces Abaaoud’s movements until last February, lists his pseudonyms, recounts minor brushes with the law and a period of detention in 2012. He heads for Turkey in January 2014, back in Belgium by the time of the Charlie Hebdo massacre, interviewed by the jihadi magazine Dabiq. It remains unclear how much of this detail was shared with France.

Arnaud Danjean, a former security official with the French defence ministry and European parliament, says that “there are Belgian files that have not been transmitted. There are problems of coordination.”

But early on Saturday the calculations and machinations of the week dissolved, somehow, into the ritual and personal solemnities of the moonlit night. At Place de la République, thousands had converged spontaneously on 5 January and their posters are still there: Je Suis Charlie, on the column atop which the statue of Marianne holds her torch aloft. Yesterday, there were hundreds only, and even their gathering was now illegal, but they hoisted the very same banner as that 11 months ago: Même Pas Peur, not even afraid.

But it had been a different week – no rallies, more fear than rage; stunned, lachrymose, frightened and frightening at every level and turn. And back at the Bataclan, perhaps the most poignant and cogent offering of all – hung by its laces, a black boot, into which was stuck a bunch of red and yellow roses, and painted on it in silver: Rock in Peace.

FRANCE’S TOUGH MEASURES

■ New clause proposed for constitution giving the executive the right to declare a ‘civil state of emergency’ in perpetuity, additional to powers forged in 1955 during Algerian war. Needs three-fifths of parliament to pass.

■ Existing state of emergency extended to three months by near-unanimous votes in both houses on Thursday and Friday. Gives powers of entry and search without warrant, strengthens oversight powers, empowers military in civilian spaces.

■ Proposal to withdraw French nationality from French-born bi-nationals convicted of terrorism-related offences, associated with extremist mosques or returned from Syria or Iraq, unless compliant with ‘return visa regime’.

■ Proposed ‘return visa regime’: French or bi-national citizens returning from Syria or Iraq would retain citizenship only if they co-operate with security services. Returnees could be immediately detained and made to wear electronic bracelets.

■ Emergency bill would allow for dissolution of extremist groups around mosques or places of prayer, blocking media glorifying terrorism (further to new legislation in June) and extended use of electronic bracelets and house arrest. It would allow off-duty police officers to carry firearms.

■ Recruitment of 5,000 new police officers and 5,000 further officers to other security services.

■ Further legislation promised by prime minister Manuel Valls for measures designed for easier capture of suspects and to extend length of time suspects can be held without filing charges.