Investigators on Sunday were probing the background of a 20-year-old Frenchman born in Russia who killed one man and wounded four other people during a stabbing spree, with sources close to the case saying he had been on a watchlist of suspected extremists.
The Saturday night attack in a lively area of theatres and restaurants near the main opera house was the latest in a series of suspected jihadist strikes in France that have killed some 245 people since 2015.
He was shot and killed by police after an officer unsuccessfully tried to stop him with a Taser.
Judicial sources said the man was born in Chechnya, a Muslim-dominated Russian republic which has been the scene of two bloody separatist wars since the 1990s.
Hundreds of Islamic militants from Chechnya have left to join terror groups in the Middle East, North Africa and other regions in recent years.
Russian news reports said the Russian embassy in Paris was pressing French officials for more information on the knifeman, whose parents have been taken into custody for questioning.
Investigators have not yet said when the man arrived in France.
The man, born in November 1997, was on France's so-called "S file" of people suspected of radicalised views who could pose security risks, the sources said, though he did not have a criminal record.
The Islamic State group claimed responsibility, saying one of its "soldiers" had carried out the attack, according to the SITE monitoring group, but provided no corroborating proof to back their assertion.
The attack was condemned by Hassen Chalghoumi, head of the Conference of French Imams.
"Last night's attack reminds us how much our security, freedom and democracy have become targets for these people," he said in a statement.
Witnesses described a wave of panic on the Rue Monsigny as people fled into bars and restaurants seeking cover as the man struck apparently at random, yelling "Allahu akbar" (God is greatest).
"I was taking orders and I saw a young woman trying to get into the restaurant in panic," Jonathan, a waiter at a Korean restaurant, told AFP.
The woman was bleeding and a young man fended off the assailant who then ran away, he said.
"The attacker entered a shopping street, I saw him with a knife in his hand," he said. "He looked crazy."
Milan, 19, said he saw "several people in distress" including a woman with wounds to her neck and leg.
"Firemen were giving her first aid. I heard two, three shots and a policeman told me that the man had been overpowered."
A 29-year-old man was killed in the attack, while a 34-year-old Luxembourg man and a 54-year-old woman were seriously wounded and rushed to hospital.
A 26-year-old woman and a 31-year-old man were slightly wounded, but Interior Minister Gerard Collomb later told reporters that all four were out of danger.
"I was on the cafe terrace, I heard three, four shots, it happened very fast," said 47-year-old Gloria.
"The bartenders told us to come inside very quickly. Then I went out to see what was going on, and then I saw a man on the ground," she added.
French President Emmanuel Macron tweeted: "France has once again paid the price in blood but will not give an inch to the enemies of freedom.
The attack again underscored the difficulty in keeping track of suspected radicalists by police facing thousands of potential risks, either homegrown or who have immigrated to the country.
Besides the S file, France maintains a File for the Prevention of Terrorist Radicalisation (FSPRT), which focuses on people judged to be terror threats.
The FSPRT watchlist alone currently has nearly 20,000 people, of whom about half are under active surveillance.
As has happened after previous suspected jihadist attacks, some lawmakers urged the government to do more to protect people from Islamic extremists.
"In the war against terrorism, words aren't enough, we need action," tweeted Laurent Wauquiez, head of the rightwing Republicains party.
France has suffered a series of major Islamist attacks including the massacre at the satirical newspaper Charlie Hebdo, the November 2015 attacks that killed 130 in Paris, and the 2016 Bastille Day truck attack in Nice that killed more than 80.
There have also been a string of less deadly but frequent attacks by lone wolf jihadists wielding knives or guns. Most of the attacks have either been claimed by the Islamic State group or been carried out in their name.
A state of emergency put in place just after the 2015 Paris attacks was lifted in October when Macron's centrist government passed a new law boosting the powers of security forces.
Thousands of French troops remain on the streets under an anti-terror operation known as Sentinelle, patrolling transport hubs, tourist hotspots and other sensitive sites.