PARIS — It has almost become routine in France: A terror attack shatters the rhythms of daily life, bringing bloodshed and anguish. The assailant turns out to be someone known to authorities. What is different now is the timing, as Paris is again on high alert, less than 36 hours before the country goes to the polls on Sunday in one of the most tumultuous and unpredictable presidential races in memory.
The brazen assault on Thursday by Karim Cheurfi, 39, a French national with a history of violence, left one police officer dead on the sidewalk of the Champs-Élysées. It has also given a potent opportunity for conservatives, primarily Marine Le Pen, leader of the far right National Front, to use the violence to try to stoke hostility toward immigrants and Muslims, as well as fears about whether citizens can be protected from terror.
Barely a week ago, with her poll numbers sagging, Ms. Le Pen tried to rally her base with a raw appeal against Muslims and immigrants. It was unclear if her gambit was resonating. Now she and other candidates are jockeying to position themselves as tough on terror, amid revelations that Mr. Cheurfi, like several attackers before him, had been on the authorities’ radar.
The Paris prosecutor’s office on Friday acknowledged having opened a preliminary terrorism investigation into Mr. Cheurfi as recently as March 9. He had been arrested in February, only to be released for lack of evidence. Following Thursday’s attack, the police found kitchen knives, a gun and a Quran in the trunk of the car he was driving, as well as pieces of paper with scribbled allegiances to the Islamic State, which claimed responsibility for the attack.
Ms. Le Pen pounced, mocking the outgoing president, François Hollande, and vowing to be an unblinkingly tough leader.
“For 10 years, under the governments of left and right, everything has been done to make us losers,” she said, speaking from her party headquarters outside Paris on Friday. “There must be a president who acts and who protects.”
But Ms. Le Pen was not the only one who stood to gain. Some analysts predicted that the principal electoral beneficiary could be the embattled mainstream center-right candidate François Fillon, who produced a book last fall called “Defeating Islamic Totalitarianism,” and who also uses harsh rhetoric to depict the antiterror fight as a war of civilizations.
Mr. Fillon, a former prime minister, and once the presidential front-runner, had languished in polls after becoming entangled in a nepotism scandal that led to embezzlement charges against him. But he has been gaining ground in recent weeks, and the attack might provide a final push.
“You can imagine a movement toward one who has held power,” said Dominique Reynié, an expert on the far right who teaches at Sciences Po. “He’s written on terrorism. He’s been prime minister.”
“For Marine Le Pen, it won’t have an effect,” Mr. Reynié said. “She’s already at the level she’s reached, partly because of terrorism.”
Throughout Friday, the authorities in Paris continued their investigation as more details became known about Mr. Cheurfi. He had been convicted of crimes four times and spent more than 10 years in prison, most of that time for shooting at police officers during a 2001 robbery, the Paris prosecutor, François Molins, acknowledged at a Friday afternoon news conference.
His arrest in February was for making anti-police threats, but the authorities lacked sufficient proof to hold him. Asked about his release, Christophe Rouget, a spokesman for one of the main police unions, said, “We live in a state of laws, after all,” citing the lack of probative evidence against him.
Even so, police made Mr. Cheurfi the subject of a preliminary terrorism investigation, Mr. Molins said.
“Cheurfi’s criminal record, his trajectory, justified the pursuit of investigations by the antiterrorism branch of my office, within a terrorism context,” he added.
A man identified by BFM-TV as Mr. Cheurfi’s lawyer described him as “extremely isolated, a solitary person,” who spent much of his time playing video games. “His development had been blocked,” said the lawyer, Jean-Laurent Panier. “His father and brother were worried about him.”
The Islamic State has claimed responsibility for the attack, but Mr. Cheurfi’s neighbors in the Paris suburb of Chelles described him as quiet, and showing no obvious signs of radicalization. “Not very friendly. Fairly proud,” said Augusto Rodriguez, a neighbor.
Mr. Cheurfi was not included among France’s notorious “S-Filers” — the thousands suspected of extremism whom the French state is officially surveilling, but does not have enough formal proof to arrest. The “S-Filers” have acquired near-mythic boogeyman status in the French imagination. On Friday, Ms. Le Pen called for their expulsion from the country, while earlier in the week, at a campaign rally in Marseille, she called them an “immense army of the shadows that wants us to live in terror.”
Emboldened after the Champs-Élysées attack, Ms. Le Pen sought, as she often does, to place the antiterrorism fight as a struggle for the French soul. The idea is at the heart of her nationalistic campaign, and even as her momentum has slowed she has still placed first in many polls before the Sunday vote. “France is targeted not for what it does, but for what it is, and the French, for the simple reason that they are French,” Ms. Le Pen said.
The French government immediately reacted harshly to Ms. Le Pen’s demands — a measure of how seriously it took the potential boost of Thursday night’s shooting to a party it views as a threat to French democracy.
Ms. Le Pen “was seeking, like after every tragedy, to take advantage of it, in order to sow division,” said the prime minister, Bernard Cazeneuve. “She’s seeking to shamelessly exploit fear and emotion for exclusively political ends.”
In the neighborhood where the attack occurred — a hybrid mix of the raffish and refined, visited by thousands of tourists daily — Ms. Le Pen’s proposals struck a chord. “She’s just gained one point in the polls. At least,” said Christophe Pohls, a barman at a cafe on the Rue de Ponthieu.
“When you see what’s going these days, sure, sure, this helps her. She wants to close the frontiers. Pay more attention to what’s going on in France,” said Mr. Pohls, who approved Ms. Le Pen’s idea of expelling those named the S-Files.
His friend Henri Martins, in charge of security at a local nightclub, agreed. “Definitely a happy coincidence for her,” he said, as he vented against what he described as the porousness of the criminal justice system.
“They arrest these guys 10 times, then they let them out?” asked Mr. Martins, an ex-policeman. “Come on. For sure, this is going to pull up her score.”
The attack occurred on Thursday night, but on Friday morning people in the neighborhood were still recovering.
“Hallucinatory. First, three shots, then four. We were in the middle of cooking, and we had to stop,” said Jean-François Desloovere, a cook at La Casita on the Rue Washington, just around the corner. “The minute you saw people running, you knew what was up.” He recalled looking down the normally packed Champs-Élysées “and it was totally empty,” with pedestrians pressing themselves against storefronts, “like flies stuck against the glass.”
At Le Carpaccio, on the Rue de Berri, terrified customers heard the bursts of gunfire, saw people running in panic down the busy street, and dove to the ground. “They went down on the ground, and broke all the glasses,” said Jamila Maachaoui, who was still sweeping up the glass on Friday morning.