PARIS -- Investigators working to understand why a 20-year-old French citizen born in the Russian republic of Chechnya went on a stabbing rampage in central Paris detained the dead suspect's parents and a friend Sunday, French officials said.

Counterterrorism investigators want to know if the assailant, identified by Chechnya's leader as Khamzat Azimov, had help or co-conspirators. The attacker killed a 29-year-old man and wounded four other people with a knife before police fatally shot him Saturday night.

The suspect was on a police watchlist for radicalism, a judicial official not authorized to speak publicly about the case told The Associated Press. But he had a clean criminal record and did not know his victims, Interior Ministry spokesman Frederic de Lanouvelle said.

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The parents were detained in Paris' northern 18th district and the friend was detained in the eastern French city of Strasbourg on Sunday afternoon, the judicial official said.

Among the injured was a 34-year-old man from Luxembourg, the foreign ministry of the small country north of France said in a statement. After emergency treatment, he was out of danger.

Witnesses reported hearing the man shouting "Allahu akbar," the Arabic phrase for "God is great," during the attack. The Islamic State of Iraq and Syria (ISIS) claimed the attacker was one of its fighters, but provided no evidence or details about his identity.

The official said the assailant, born in 1997, had French nationality but was born in the largely Muslim Russian republic of Chechnya, where extremism has long simmered. The official wasn't authorized to be publicly named speaking about an ongoing investigation.

French Interior Minister Gerard Collomb, third from the left, gestures as he addresses a meeting with Chiefs of Staff at The Ministry of Interior in Paris on May 13, 2018. Getty

The attacker targeted five people and then fled, according to Paris police and a witness. When police officers arrived minutes later, he threatened them and was shot dead, according to police union official Yvan Assioma. Several witnesses said the attacker ran directly toward police.

Bar patrons and opera-goers described surprise and confusion, and being ordered to stay inside while the police operation was underway on rue Monsigny in the lively 2nd arrondissement, or district, of the French capital.

"I was working in the restaurant and suddenly I heard a woman screaming ... he came and attacked her," said Jonathan, a witness working nearby who wouldn't provide his last name. "That's when the panic started, everyone started screaming and trying to reach our restaurant. ... The attacker just kept walking around with his knife in his bloodied hands."

"Police were quickly on the scene, in less than five minutes. They encircled him and he tried to attack them with a knife but they shot him down," he told reporters.

A 23-year-old Brazilian tourist from Washington told The Associated Press that she and her aunt saw the fatally wounded victim on the ground, watched the ambulance arrive and rescue workers try to revive him.

"They kept doing CPR for what seemed to be a really long time and, in the end, he passed, and they put the sheet over him," said Carolina Melo. She said she and her aunt had been in their hotel, went out to investigate the noisy confusion nearby but retreated after hearing several shots of gunfire. The scene, she said, "was kind of a mess."

A police official said the assailant didn't have identity documents with him during Saturday's attack but was identified thanks to DNA.

The four people injured are out of life-threatening danger, Collomb, the interior minister, said. Collomb held a special security meeting Sunday to address the Paris attack, whose motive was unclear.

President Emmanuel Macron tweeted Saturday expressing condolences to victims and saying France will not cede to "enemies of freedom." "France is once again paying the price of blood, but will not cede an inch to enemies of freedom," Macron said in French.

President Trump took to Twitter on Mother's Day to address the incident, writing "This kind of sickness & hatred is not compatible with a loving, peaceful & successful country [Paris]":

So sad to see the Terror Attack in Paris. At some point countries will have to open their eyes & see what is really going on. This kind of sickness & hatred is not compatible with a loving, peaceful, & successful country! Changes to our thought process on terror must be made. — Donald J. Trump (@realDonaldTrump) May 14, 2018

ISIS' Amaq news agency said the assailant carried out the attack in response to the group's calls for supporters to target members of the U.S.-led anti-IS military coalition. France's military has been active in the coalition since 2014, and ISIS adherents have killed more than 200 people in France in recent years.

The Russian Embassy in France said that they have asked French authorities for more information on the attacker. Some refugees fleeing wars in Chechnya in the 1990s and early 2000s settled in France, but this country has not seen a high-profile attack by Chechens in the past. Two Chechen brothers were behind the deadly bombing of the Boston Marathon in 2013.