A gunman killed a police officer and seriously wounded two others before being killed himself in Paris’s Champs-Elysees shopping district Thursday, in what police said was likely a terror attack.

Paris police spokeswoman Johanna Primevert told The Associated Press that the attacker targeted police guarding the area, near the Franklin Roosevelt subway station at the center of the avenue popular with tourists.

French Interior Ministry spokesman Pierre-Henry Brandet said on BFM television that a man came out of a car and opened fire on a police vehicle.

A French government spokesman said the assailant was armed with an automatic firearm akin to a “war weapon.”

Reuters quoted a police source who said there were two assailants. The second attacker escaped, reports said, but Brandet said it was too early to say whether the attacker had an accomplice, and noted that authorities were studying multiple potential motives.

Two police officials, speaking on condition of anonymity, said the assailant had been flagged as an extremist. French prosecutors have opened a terrorism investigation into the attack.

Police and soldiers sealed off the area around the Champs-Elysees after the shooting, ordering tourists back into their hotels and blocking people from approaching the scene.

Firefighters and rescuers stand by the site of a shooting on the Champs Elysees in Paris on April 20, 2017. (AFP Photo/Franck Fife)

Firefighters and rescuers stand by the site of a shooting on the Champs Elysees in Paris on April 20, 2017. (AFP Photo/Franck Fife)

Emergency vehicles blocked the wide avenue, which cuts across central Paris between the Arc de Triomphe and the Tuileries Gardens, normally packed with cars and tourists, and subway stations in the area were closed off.

US President Donald Trump was quick to react to the incident, saying it appeared to be “another terrorist attack.”

“Our condolences from our country to the people of France,” Trump said during a press conference with Italian Prime Minister Paolo Gentiloni. “It is a very, very terrible thing that’s going on in the world today, but it looks like another terrorist attack. What can you say? It just never ends. We have to be strong and we have to be vigilant and I have been saying it for a long time.”

A Paris resident said the gunfire sent scores of tourists fleeing into side streets. Badi Ftaiti, a Tunisian-born mason who has spent three decades in Paris, said the attack didn’t panic him.

But the 55-year-old said visitors to the city “were running, running… Some were crying. There were tens, maybe even hundreds of them.”

Police officers block the access to the Champs Elysees in Paris after a shooting on April 20, 2017. (AFP Photo/Franck Fife)

Police officers block the access to the Champs Elysees in Paris after a shooting on April 20, 2017. (AFP Photo/Franck Fife)

The attack came three days before the first round of France’s tense presidential election. Security is high around the vote.

The incident also comes just two days after police arrested two men in southern Marseille with weapons and explosives who were suspected of preparing an attack to disrupt the first-round of the presidential election on Sunday.

France is in a state of emergency and at its highest possible level of alert since a string of terror attacks that began in 2015, which have killed over 230 people.

Thousands of troops and armed police have been deployed to guard tourist hotspots such as the Champs-Elysees and other potential targets like government buildings and religious sites.