PARIS — The Champs-Elysees gunman who shot and killed a police officer just days before France's presidential election was detained in February for threatening police but then freed, two officials told The Associated Press on Friday. He was also convicted in 2003 of attempted homicide in the shootings of two police officers.

The French government pulled out all the stops to protect Sunday's vote as the attack deepened France's political divide.

"Nothing must hamper this democratic moment, essential for our country," Prime Minister Bernard Cazeneuve said after a high-level meeting Friday that reviewed the government's already heightened security plans for the two-round presidential vote that begins Sunday.

"Barbarity and cowardice struck Paris last night," the prime minister declared, appealing for national unity and for people "not to succumb to fear."

Investigators believe at this stage that the gunman, 39-year-old Frenchman Karim Cheurfi, was alone in killing one police officer and wounding two others and a female German tourist on Thursday night, a French official who discussed details of the investigation with the AP said on condition of anonymity.

The attack came less than 72 hours before the polls open.

Police shot and killed Cheurfi after he opened fire on a police van on Paris' most famous boulevard. Investigators found a pump-action shotgun and knives in his car. Cheurfi's identity was confirmed from his fingerprints.

Cheurfi had been detained toward the end of February after speaking threateningly about police but was then released for lack of evidence, according to that French official and another, who also spoke on condition of anonymity because they weren't authorized to publicly discuss the probe.

The policeman killed Thursday was identified as Xavier Jugele by Flag!, a French association of LGBT police officers. Its president, Mickael Bucheron, told AP the slain officer would have celebrated his 38th birthday at the beginning of May.

Jugele was among the officers who responded to the gun-and-bomb attack on Paris' Bataclan concert hall on Nov. 13, 2015, among a wave of assaults in the French capital that killed 130 people, he told People.com .

He was also there a year later when the venue reopened with a concert by Sting, saying how happy he was to be "here to defend our civic values."

"This concert's to celebrate life. To say 'No' to terrorists," the media outlet quoted Jugele as saying.

The Islamic State group claimed responsibility for Thursday's attack in an unusually quick statement that sowed confusion by apparently misidentifying the gunman.

Municipal workers in white hygiene suits were out before dawn to wash down the sidewalk where the assault took place — a scene now depressingly familiar after multiple attacks that have killed more than 230 people in France over two years. Delivery trucks were out on early morning rounds. Everything would have seemed normal if not for a row of TV trucks parked along the boulevard that is a must-visit for tourists.

A key question was how the attack might affect French voters, since campaigning is banned starting Friday at midnight.

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."

The two top finishers Sunday advance to a winner-takes-all presidential runoff on May 7. Two of the main candidates, conservative Francois Fillon and centrist Emmanuel Macron, canceled campaign events Friday.

The attack brought back the recurrent campaign theme of France's fight against Islamic extremism, one of the mainstays of the anti-immigration platform of far-right leader Marine Le Pen and also, to a lesser extent, of Fillon.

Le Pen, speaking at her campaign headquarters, urged the outgoing Socialist government to immediately re-establish border controls. Cazeneuve, the Socialist prime minister, accused the National Front leader of seeking to make political hay from the assault.

After Le Pen spoke scathingly Friday of the government's fight against extremism, Cazeneuve noted that Le Pen's party in 2014 voted against an anti-terrorism law and, in 2015, against a law that beefed up resources for French intelligence services.

He said: "She seems to be deliberately forgetting everything that has been done over five years to make people forget that she opposed everything, without ever proposing anything serious or credible."

Fillon, for his part, pledged to maintain the state of emergency that has been in place since the November 2015 attacks.

"The fight for the French people's freedom and security will be mine. This must be the priority," he said.

Asked if the assault would impact voting, the centrist Macron said "no one knows" and appealed for cool heads.

"What our attackers want is death, symbolism, to sow panic (and) to disturb a democratic process," the 39-year-old former investment banker said.

Macron said he canceled campaign stops out of a sense of "decency" and to allow police to concentrate resources on the investigation. Said by polls to be running neck-and-neck with Le Pen, he tore into her claims that previous attacks wouldn't have happened under her watch.

"She won't be able to protect our citizens," Macron said.

The two police officers injured in the attack are out of danger, Interior Ministry spokesman Pierre-Henry Brandet said. National police spokesman Jerome Bonet, also speaking on BFM television, said thousands of people were out on Paris' iconic boulevard when the gunman opened fire and that the rapid response of officers who shot and killed him avoided possible "carnage."

Elena Worms, who was walking her dog near the Champs-Elysees, called the attack "destabilizing" and said she fears it will "push people to the extremes." She said her plans to vote for Fillon, a former prime minister, remain unchanged.

Investigators searched a home early Friday in an eastern suburb of Paris believed linked to the attack and detained three of the gunman's family members for questioning.

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. It recalled two recent attacks on French 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.

Sylvie Corbet, Angela Charlton and Raphael Satter in Paris, Jeff Schaeffer and Nadine Achoui-Lesage in Chelles, France, and Raf Casert in Brussels contributed to this report.