A gunman opened fire on a French police vehicle on the famous Champs-Élysées avenue in Paris on Thursday, killing one officer and injuring three others, authorities said.
"A [police] car parked on the #ChampsElysees was targeted by an individual's gunfire," the Ministry of Interior said on Twitter. "The individual was shot dead by law enforcement."
French President François Hollande addressed the nation Thursday night, saying the French security forces would be working to investigate the motive and any accomplices who may have played a role.
"We believe the attack is of a terrorist nature," Hollande said.
French officials have not made any public statement linking the terror group to the shooting. But Site Intel Group, which monitors insurgent networks, said ISIS claimed responsibility for the attack through its news agency, Amaq.
ISIS has claimed responsibility for attacks where the suspects were found to have no direct link to the group. On Thursday, however, the claim of responsibility from the terrorist group came much more quickly than in the past.
The pseudonym listed for the alleged attacker, according to ISIS, is Abu Yusuf al-Beljiki — meaning "the Belgian" in Arabic — suggested he had some sort of tie to the group, the Associated Press reported.
But unnamed officials told the AP that they had identified the shooter as 39-year-old Karim Ceurfi.
The officials told the AP that Cheurfi had been detained in February for threatening police, but was eventually let go due to a lack of evidence.
In 2003, he was also convicted of attempted homicide during a shooting in which he targeted two police officers, the AP reported.
Belgium's Interior Minister Jan Jambon told public broadcaster VRT on Friday morning that Cheurfi is believed to be a French national.
A Belgian police official told BuzzFeed News late on Thursday that a possible suspect or accomplice had turned himself into police and is being investigated for any links to the attack in Paris.
In his address, Hollande also expressed his condolences for the slain officer and two other officers who were injured in the confrontation.
A bystander was also wounded in the shootout.
"Our security is at stake," Hollande said. "This is the message I would like to send tonight. This is a message not only to police forces, the army, it's a message to everyone: you will be protected."
France's Interior Minister Matthias Fekl touted the work of police at the scene, and said their quick action avoided more injuries.
"We've avoided a bigger tragedy on the Champs-Élysées," Fekl told reporters. "Tonight we avoided a bloodbath that could have been extremely dire."
A woman named Véronique told BuzzFeed News that she was having a drink at a bar near the Champs-Élysées Thursday night when the shooting took place.
She initially thought the gunshots were the sound of firecrackers, but then realized the noise was too violent. She said that soon after the shots rang, people went inside the bar and took refuge on the floor.
After more noise, she said, the Champs-Élysées had "frozen," and that once police arrived, they ordered people to quickly leave the area.
But when she and others sough safety inside other establishments, "Restaurants closed and did not let us in."
Eventually, Véronique, who did not give her last name for security reasons, found a taxi and went home.
Ministry spokesman Pierre-Henry Brandet told BFM television that a man exited a car and opened fire on the police vehicle, "deliberately" targeting the officers with an automatic weapon.
There was "no other police operation underway" in the popular shopping district, he said.
Brandet told reporters the shooter's background and motive had not immediately been determined, but that France remained on high alert because of past terror threats.
Brandet confirmed that counterterrorism investigators have been called to look into the shooting, and that the country remains on high alert.
"If this is considered to be a terrorist attack, this is a serious crime and there may be some accomplices to be interrogated," he said.
Overnight, French prosecutor François Molins said investigators had determined the identity of the gunman and were conducting searches throughout Paris, the AP reported.
Dozens of police cars and helicopters descended on the popular tourist strip following the shooting, with police urging the public to avoid the area.
Anti-terror prosecutors had also reportedly mobilized to investigate the incident.
France remains in a state of emergency and security forces have been out in greater force in Paris in the wake of deadly Islamic extremist attacks over the years.
"Tribute to the slain police officer at the Champs-Elysées tonight," French Prime Minister Bernard Cazeneuve wrote on Twitter. "Solidarity to [the officer's] injured colleagues and their loved ones."
French voters are set to go to the polls on Sunday in the first round of the country's presidential election. Cazeneuve said that the country's security forces are fully mobilized, and nothing would stop the elections going ahead, Reuters reported.
The candidates for the presidency were engaged in a televised debate as the attack took place.
While speaking at the White House alongside the Italian prime minister, US President Donald Trump offered condolences to the people of France.
"It looks like another terrorist attack," Trump said. "What can you say? It just never ends. We have to be strong and we have to be vigilant."
On Twitter Friday morning, Trump went on to say, "Another terrorist attack in Paris. The people of France will not take much more of this. Will have a big effect on presidential election!"
The US State Department, meanwhile, advised people to avoid the area and monitor local news.
"Authorities are telling people to avoid the area after a shooting," the department tweeted.
Mitch Prothero contributed reported for this article from Brussels.