Paris - A known terror suspect shot dead a French policeman and wounded two others on Thursday on Paris's Champs Elysees in an attack claimed by the Islamic State group, days before a presidential election.
As the search intensified, it emerged that Cheurfi had a history of violent crime and was convicted of shooting at police in 2001.
The shooting has not officially been declared a terrorist act but anti-terrorist forces are leading the investigation, French President François Hollande said.
Two French officials said the gunman was detained towards the end of February after speaking threateningly about the police, but he was then released due to a lack of evidence.
The Islamic State group quickly claimed responsibility for the attack that killed one police officers and injured two others on the iconic Parisian boulevard Thursday night.
Police shot and killed Cheurfi after he opened fire on a police van on Paris' most famous boulevard.
In addition to the assault rifle used in the attack, he had a pump action shotgun and knives in his vehicle, the sources said. Cheurfi's identity was confirmed from his fingerprints.
Earlier this week, in a separate incident, French authorities arrested two men in Marseille, saying they were suspected of planning an "imminent" attack ahead of the election and were found in possession of an arsenal of weapons.
The risk for the main presidential candidates is misjudging the public mood by making an ill-perceived gesture or comment.
Inserting himself into the debate, U.S. President Donald Trump tweeted that the attack "will have a big effect" on the election and that "the people of France will not take much more of this".
Fillon, Le Pen and independent centrist Emmanuel Macron canceled planned campaign events after the shooting. Cazeneuve, the Socialist prime minister, accused the National Front leader of seeking to make political hay from the assault. Officials haven't said if they believe that claim is credible, but they believe he was operating alone.
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"The fight for the French people's freedom and security will be mine. This must be the priority", he said. Security has been especially high since Tuesday, when police said they thwarted a terror attack by arresting two men.
Controls on immigration and national security are cornerstones of Le Pen's National Front agenda and on Friday she said she would reinstate border checks and expel foreigners who are on watch lists of intelligence services.
If Melenchon makes it to the runoff, he is projected to beat both Le Pen and Fillon by comfortable margins although he is seen losing to Macron 41 percent to 59 percent.
The attack on Thursday night on the Champs Elysees boulevard added a new source of unpredictability to a closely contested election that will decide the management of France's 2.2 trillion euro economy, which vies with Britain for the rank of fifth largest in the world.
Fillon said that if elected he would focus on the destruction of ISIS.
In an announcement through its propaganda arm Amaq, the terror group named the gunman as Abu Yusuf al Beljiki and suggested he was a Belgian national.
Matthias Fekl, the French interior minister, paid tribute to the dead policeman and praised his colleagues who he said had prevented a bloodbath.
Investigators searched a home early Friday in an eastern suburb of Paris believed linked to the attack and police detained for questioning three of the gunman's family members - routine in such cases.
The attack appeared to fit a pattern of European extremists targeting security forces and symbols of state to discredit, take vengeance on or destabilize society.
The assault recalled two recent attacks on soldiers providing security at prominent locations around Paris: one at the Louvre museum in February and one at Orly airport last month.
For Sunday's presidential vote, the government is mobilizing more than 50,000 police and gendarmes to protect the 70,000 polling stations, with an additional 7,000 soldiers also on patrol.