Daesh "the Islamic State extremist organization" claimed responsibility for the shooting death of a French policeman on Paris' Champs-Elysees on Thursday, three days before the first round of the country's presidential election.
The organization's Amaq news outlet said that the perpetrator, whose alias suggested he was from Belgium, was a member of the group.
Paris prosecutor Francois Molins said the attacker had been identified though he refused to give any further details, saying investigators wanted to determine whether he had any accomplices.
The attacker pulled up beside a police car on the central Paris avenue and opened fire, killing one of the officers in the vehicle, Interior Ministry spokesman Pierre-Henry Brandet said.
He then tried to escape on foot, still firing, and injured another two officers seriously before being shot dead himself. A foreign tourist also suffered minor injuries.
But Brandet noted that there had only been the one incident in Paris, not multiple ones, as in the 2015 terrorism attacks in the city which were also claimed by Islamic State.
"There was only one incident of this kind this evening in Paris, there were no others," he said, denying reports that a second policeman had died in the attack.
The area was sealed off by police and bomb disposal experts started to examine the suspect's car, although it was not clear if that was due to specific concerns or was a routine precaution.
President Francois Hollande, speaking minutes before the Islamic State claim was published, said he was convinced that "the paths for the investigation... are of a terrorist nature."
Anti-terror prosecutors took over the probe shortly after the attack, which took place at about 9 pm local time (1900 GMT) near the Arc de Triomphe, an area popular with tourists.
"A national tribute will be paid to this police officer who was murdered in such a cowardly manner," Hollande said.
The president announced that he had summoned a meeting of the defence and national security council for 8 am Friday.
"We will be absolutely vigilant, in particular with regard to the electoral process," he said.
At the scene, police sirens wailed and helicopters circled. One man, who said he had been dining in a restaurant on the Champs-Elysees and didn't want to give his name, said he had heard the shots fired.
"Lots of people ran up to the first floor," he said. "Some were panicking." Another man said the attacker had deliberately attacked the officers.
"How are we supposed to vote with this kind of danger?" asked another woman.
Police later searched a flat where the perpetrator lived in a suburb of Paris, according to sources from the investigation.
The news agency AFP reported that the man had already been the target of an anti-terrorism investigation after expressing an intention to kill police officers. It also said he was a 39-year-old French man.
The shooting came only two days after authorities announced that they had foiled an "imminent" terror attack planned for the days before the election.
Police on Tuesday arrested two men and seized guns, explosives and an Islamic State flag in a raid in Marseille.
One of the suspects had allegedly attempted to send members of Islamic State a video pledging allegiance or claiming an attack.
The video included a shot of a newspaper front page concerning one of the candidates in Sunday's presidential vote, according to prosecutors.
About 50,000 police and troops are due to be deployed to secure the polls.
A state of emergency imposed after repeated deadly attacks by Islamist extremists over the past two years is still in place.
Thursday's killing quickly drew condemnation from the candidates in the presidential election, who were being interviewed one by one on France 2 television as the events unfolded.
Centrist Emmanuel Macron, favoured in the polls, said it showed that the terrorist menace was "an unknown quantity that will be with us through the coming years."
US President Donald Trump said the killing of a police officer appeared to be a terrorist attack.
Trump and Italian Prime Minister Paolo Gentiloni offered condolences to the people of France during a joint press conference after talks at the White House.
"It looks like another terrorist attack," Trump said. "What can you say? It just never ends. We have to be strong and we have to be vigilant."
German Chancellor Angela Merkel also expressed her condolences to the victims and their families, according to her spokesman Steffen Seibert.
Attacks claimed by Islamic State, notably in Paris and Nice, claimed some 238 lives in France in 2015 and 2016.
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