A policeman was shot dead and two others were seriously wounded on the Champs-Élysées in Paris on Thursday night, in an attack claimed by Islamic State.
The suspect, who was shot dead by French police, was the subject of a "Fiche S" surveillance file and was on the radar of the French domestic security service DGSI, the source said.
On Twitter, the police union has said the officer was killed while in a vehicle stopped at a red light, by an attacker driving past.
Meanwhile, as France's elite anti-terrorism took control of the crime scene and searched the shooter's vehicle for possible explosives dozens of police officers kept the curious away.
Police appealed to the public to remain out of the area as officers descended on the popular shopping area.
France remains under a state of emergency, after a series of deadly terrorist attacks over the past several years.
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Far-right candidate Marine Le Pen took to Twitter to offer her sympathy for law enforcement officers "once again targeted".
The French Interior Ministry said the attack deliberately targeted police officers guarding the area.
The Avenue des Champs-Élysées is an avenue in the 8th arrondissement of Paris, 1.9 kilometres long and 70 metres wide, running between the Place de la Concorde and the Place Charles de Gaulle, where the Arc de Triomphe is located.
The incident came as French voters prepared go to the polls on Sunday in the most tightly-contested presidential election in living memory.
The world-famous avenue has been sealed off and nearby stations on the Paris Metro have been closed.
Authorities asked people to stay away from the area. "It just never ends".
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"We're three days before a presidential election", Eleanor says. Earlier this week, police in Marseille arrested two men who were allegedly plotting an attack before the election, per Reuters.
The incident 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.
One witness told French television station BFM that she heard a shooting, saw a man's body on the ground and the area was quickly evacuated by police.
In Washington, President Donald Trump said it "looks like another terrorist attack".
None of the candidates immediately commented.
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