French prosecutors have opened a terrorism investigation into an attack in the Champs Elysees shopping district in Paris where an armed man killed one policeman and seriously injured two people.

President Francois Hollande said he was "convinced" a "terrorism" investigation was the correct approach to the incident before the Islamic State of Iraq and the Levant (ISIL) group claimed responsibility for Thursday's attack via its Amaq website.

Hollande promised "absolute vigilance, particularly with regard to the electoral process" as France braces itself for the first round of a presidential election on Sunday.

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French television networks reported that the attacker was a 39-year-old French national and was known to anti-terrorism police.

Police said the attacker had been convicted in 2005 of three counts of attempted murder - two of them against police officers.

Raids took place at his address in a suburb to the east of Paris.

"The identity of the attacker is known and has been checked. I will not give it because investigations with raids are ongoing," Francois Molins, Paris prosecutor, said.

"The investigators want to be sure whether he had or did not have accomplices."

Joseph Downing, a researcher at the London Schools of Economics, said that if the attacker was, as reported, known to police, it raised questions of how he could still stage an attack.

"This is something we've seen repeatedly in France, that everyone who has popped up to carry out an attack, has been on the police database," he told Al Jazeera.

"Even the person that murdered the priest [in Normandy] last year was actually on bail for terror offences at the time."

Potential beneficiaries

In another development, France's interior ministry said a man sought by the authorities turned himself in to police in Belgium on Friday, adding that it was "too early to say" if he was linked to the shooting.

Citing a source, the AFP news agency reported that the 35-year-old was described as "very dangerous" and had been sought by Belgian police as part of a separate investigation.

Al Jazeera's Natasha Butler, reporting from Paris, said the attack could benefit Marine Le Pen, the National Front leader, and Francois Fillon, the conservative presidential hopeful.

"The attack has thrown the spotlight back on to security and people will have that firmly on their minds when they head to the polls on Sunday," she said.

"Le Pen and Fillon have both taken a very hard line on security during their campaigns. They both link immigration to terrorism, with Le Pen going as far as saying she will ban it altogether. So both will certainly be playing up that narrative when they speak later on Friday."

Several candidates announced after the attack that they had ended their campaigns early as a mark of respect.

While Thursday's attack shocked France and reverberated across the world, Al Jazeera's Shafik Mandhai, reporting from Paris, said life in the city had returned to normal on Friday morning.

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"The only notable difference to a normal day on the Champs Elysees are the satellite trucks of various media outlets parked in the area," he said.

"The police presence is also notably low key."

France has been under a state of emergency since 2015 and has suffered a series of attacks that have killed more than 230 people in the past two years.

The Charlie Hebdo magazine was hit in January 2015, sites around Paris including the Bataclan concert hall were targeted in November the same year, and families at a fireworks display in Nice in July 2016.

'We were trapped'

Paul Gray, a US citizen working in Paris, told Al Jazeera that it was the second time he found himself caught up in an attack.

"It was crazy, I work nearby and we were having a few beers with a colleague who was leaving and we just saw people running," he said.

"We were trapped, it was traumatic because the same thing happened to me the last time there was an attack."

Al Jazeera's Ahmed El Amraoui, also reporting from Paris on Friday, said: "The most alarming fact is the significant presence of home-born radicalised people in France.

"Marginalisation and lack of opportunity can be considered as the main factors and fertile platforms for terrorism to grow, as most of the attackers came from marginalised and poor areas.

"The rule of law and freedom alone do not change much in the lives of people if they are not supported by inclusive growth."