PARIS (Reuters) - Police on Sunday scoured the background of a Chechnya-born Frenchman who killed a man in a knife attack in Paris, questioning the parents and a friend of the 21-year old, who had been flagged previously as a potential security risk.

The assailant had shouted “Allahu akbar” (God is greatest) as he began his stabbing rampage late on Saturday. He fatally knifed a 29-year-old man and wounded four others, among them a Chinese and a Luxembourg citizen, before police shot him dead.

A judicial source named the attacker as Khamzat A, without giving his full name, which BFM TV and other French media said was Azimov.

The attack took place in the bustling Opera district, known for its many restaurants, cafes and the Palais Garnier opera.

It was the latest in a succession of attacks in France since January 2015 in which more than 240 people have been killed.

Since 2016 the attacker had been on a counter-terrorism watchlist of suspected radicals who may be a threat to national security, government spokesman Benjamin Griveaux said.

The stabbing again exposed the difficulty European intelligence services face keeping track of suspected extremists and countering the threat posed by homegrown militants and foreign jihadists.

France has participated in a U.S.-led coalition battling Islamic State in Iraq and Syria, and it also intervened in Mali to push back an Islamist rebellion in the West African state.

Its military interventions overseas have exposed it to attack by Islamist militants at home.

Islamic State claimed responsibility for Saturday’s attack, but provided no proof. Griveaux said the claim had not yet been fully authenticated.

In a video which the SITE intelligence monitoring group said was posted by Islamic State’s Amaq news agency, a young man described as the attacker pledged allegiance to Islamic State leader Abu Bakr al-Baghdadi.

French police secure a street after a man killed a passer-by in a knife attack in the heart of Paris and injured four others before being shot dead by police, according to French authorities in Paris, France, May 12, 2018. REUTERS/Lucien Libert

Speaking in French, his face largely obscured by a black hood and scarf, he said “infidels” fighting Islamic State were to blame. “You started it by killing Muslims,” he said.

It was not immediately possible to confirm the identity of the man in the video.

The assailant became French when his mother obtained citizenship in 2010, Griveaux said in a joint interview with broadcasters LCI and RTL and newspaper Le Figaro.

He rejected criticism from opponents of President Emmanuel Macron that the government was not doing enough to stem such attacks, saying: “Zero risk does not exist.”

Judicial sources said the assailant’s parents as well as a friend of his were being held for questioning. The friend, arrested in the eastern French city of Strasbourg, was born in 1997, a source said.

BFM TV said the attacker had long lived in Strasbourg before moving to Paris last year.

A local elected official told Reuters that about 40 to 50 Chechen families lived in that area of Strasbourg, not mixing much with other people.

“I’ve been sounding the alarm since 2008-2009. This is fertile ground for radicalization,” Eric Elkouby, a Socialist, said of the area, which he said was impoverished and had been neglected by authorities.

GRAPHIC - Knife attack in Paris: tmsnrt.rs/2KePel5

‘WE HID UNDER THE TABLE’

In Washington, U.S. President Donald Trump said on Twitter that countries needed to “open their eyes” and change the way they view such attacks.

“So sad to see the Terror Attack in Paris,” Trump tweeted. “This kind of sickness & hatred is not compatible with a loving, peaceful, & successful country! Changes to our thought process on terror must be made.”

The White House issued a statement earlier on Sunday that condemned the attack and expressed solidarity with the French nation.

Police union representative Rocco Contento told Reuters the attacker stabbed bystanders before rushing at police on Saturday evening shouting “I will kill you, I will kill you!”

He was then shot by the officers.

“It was scary,” said Emma Klibbe, a 32-year old Australian who was waiting to get into a nearby restaurant and saw a man walk by who had been wounded in the attack.

“We heard someone shout and then a woman screamed ‘run inside!’ We ran inside and hid under the table,” said Klibbe, who teaches English in Paris.

The four people wounded were out of danger, Interior Minister Gerard Collomb told reporters.

A picture seen by Reuters showed a bare-chested and bearded young man dressed in black sweatpants lying on the ground and being helped by emergency services. A source said he was the attacker.

“We heard gun fire, two gun shots,” said waitress Sarah Gousse. “The street was just filled with police and firemen and they tried to revive him (the attacker).”

In a similar incident in October, a man stabbed two young women to death in the port city of Marseille before he was shot dead by soldiers.

The deadliest of the attacks that have hit France over the past three years occurred in Paris in November 2015, when 130 people were killed.