Investigators are trying to find out whether the man who shot dead a policeman in Paris had accomplices as he had shown no previous signs of radicalisation despite a long prison record.
The gunman, identified as Karim Cheurfi, opened fire on a police vehicle parked on the Champs Elysees in Paris late on Thursday, killing one officer and injuring two others before being shot dead.
The attack, which was claimed by the Islamic State terror group (ISIS), overshadowed the last day of campaigning for Sunday's presidential election first round.
Cheurfi, 39, a French national who lived with his mother in the eastern Paris suburb of Chelles, had spent some 14 years in prison from 2001 for crimes including gun attacks on law enforcement officers.
"The investigations will now focus on determining ... the potential help that he may have benefited from," Paris prosecutor Francois Molins told a news conference on Friday.
"He was not on the security watch list and had shown no signs of radicalisation despite his many years in prison."
But Mr Molins confirmed police had found a note with handwritten messages defending ISIS near his body, addresses of police establishments in his car and a Koran.
Police believe he had "opened fire on the officers in the knowledge he would be killed by them", a source close to the investigation said.
As well as the assault rifle used in the attack, a pump action shotgun and knives were in his car, Mr Molins said. Three of his family have been placed in detention, the French interior ministry said.
The car belonging to an attacker and a police van are pictured on the Champs Elysees. (AAP)
A tourist is blocked by French soldiers next to the Champs Elysee Avenue during ongoing police operations. (AAP)
Frightened onlookers hold their hands in the air. (AAP)
Cheurfi served 10 years in prison after firing on two plain-clothes officers in 2001 as they tried to apprehend him in a stolen car. While in detention, he shot and wounded a prison officer after seizing his gun.
Released on probation in 2015 from a further two-year jail term imposed for lesser offences, Cheurfi was arrested again in February after threatening to kill police officers - but released for lack of evidence.
A French interior ministry spokesman initially confirmed on Friday that a second man was being sought, based on information from Belgian security services.
"It's too early to say how or whether he was connected to what happened on the Champs Elysees," ministry spokesman Pierre-Henry Brandet said. "There are a certain number of leads to check. We are not ruling anything out."
A potential second suspect was identified as Youssouf El Osri in a document seen by Reuters. Belgian security officials had warned French counterparts before the attack that El Osri was a "very dangerous individual en route to France" aboard a high-speed train.
Mr Brandet later said a man with that name had turned himself in at a police station in Antwerp.
ISIS claimed responsibility for the shooting soon afterwards, in a statement identifying the attacker as "Abu Yousif al-Belgiki (the Belgian)".
El Osri's connection with either the downed assailant or the man named by ISIS remained unclear on Friday.
"We don't understand why Islamic State has identified the wrong person," said a police source. "What does seem clear is that Islamic State was planning something."
Observers had long feared an attack ahead of Sunday's election in France, following a string of atrocities by extremists since 2015 that have claimed over 230 lives.
The impact on the outcome of one of the most unpredictable contests in decades is unclear, but far-right leader Marine Le Pen and scandal-hit conservative Francois Fillon immediately cancelled their campaign events on Friday.
The shooting came two days after the arrest of two men in southern Marseille who allegedly possessed weapons and explosives and were suspected of preparing an attack to disrupt the campaign which concludes on Sunday.
French President Francois Hollande, again forced to address the nation after an attack, promised "absolute vigilance, particularly with regard to the electoral process" and paid tribute to the police.
Le Pen earlier welcomed security moving to the heart of the campaign Thursday as she took part in a prime-time interview show alongside 10 other presidential candidates.
"We are suffering the consequences of a laxity that has continued for years," she said shortly before the shooting, promising to take a hard line against extremists and anyone suspected of being an Islamist.
For weeks, centrist former banker Emmanuel Macron and Le Pen have been out in front but opinion polls now show there is a chance that any of four leading candidates could reach the second-round runoff on May 7.
As the first details of the Champs Elysees shooting filtered through, US President Donald Trump sent his condolences and said that "it looks like another terrorist attack. What can you say? It just never ends."
France is in a state of emergency and at its highest possible level of alert since the attacks that began in 2015.
The Charlie Hebdo magazine publisher was hit in January 2015, sites around Paris including the Bataclan concert hall were targeted in November the same year and a truck was deliberately driven through crowds at a fireworks display in Nice in July last year.
In between, there have been a series of smaller attacks, often aimed at security forces.
Thousands of troops and armed police have been deployed to guard tourist hotspots such as the Champs Elysees or other potential targets including government buildings and religious sites.
In February, a man armed with a machete in each hand attacked soldiers on patrol at Paris's Louvre Museum. The attacker, a 29-year-old Egyptian, was seriously injured.
And in March, a 39-year-old man was killed at Paris's Orly airport after attacking a soldier.
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