The Paris prosecutor's office said the friend, identified as Abdul Hakim A., is accused of association with terrorist criminals with plans to attack people. He was put in provisional detention pending further investigation.
France's anti-terrorism prosecutor Francois Molins said in his first news conference since the Saturday night attack that two female friends of the men were arrested earlier in the day in the Paris region. He did not elaborate.
Khamzat Azimov, who was born in the Russian republic of Chechnya, killed one person and wounded five others with a knife in the rampage through streets near the Paris Garnier Opera house. He was killed by police as he advanced on them, saying "Shoot! Shoot! Shoot!"
President Emmanuel Macron was meeting Thursday with the three police officers who intervened during the attack.
Molins said Azimov's parents, who were detained shortly after the stabbing spree, recognized their son in a video declaring allegiance to the Islamic State group that was released a day later.
In the posthumous video, released by Islamic State group's Aamaq news agency, Azimov called on Muslims in Europe to "take action in the land of disbelievers" if they can't travel to the crumbling caliphate in Iraq and Syria, which has been pounded by coalition forces. He said French citizens should pressure their government "if you want it (attacks) to end."
Azimov had French and Russian citizenship, Molins said. His friend Abdul Hakim A. also was from Chechnya.
Azimov and Abdul Hakim A. - both 20 - were on a watch list for potential radicalization, Molins said. Abdul Hakim A. was placed on a second alert list in October for prevention of radicalization with a terrorist character.
Azimov was placed on the list containing thousands of names because of his links to a young woman that Abdul Hakim A. married before she tried to go to Syria.
The prosecutor said Abdul Hakim A., arrested a day after the attack in Strasbourg where Azimov had previously lived, denied contacting or seeing his friend for several months. The Azimov family had moved to the northern 18th district of Paris last July.
Investigators detained Abdul Hakim A. because of his marriage to the young woman headed to Syria, Molins said.
"Abdul Hakim A. came to Paris from Jan. 8-12, 2017, accompanied by Khamzat Azimov, in order to get a religious wedding with this young woman, the eve of her departure for Syria," the prosecutor said.
Studying Abdul Hakim A.'s communications, Molins said investigators noted much activity on encrypted networks, including in the night after Azimov's rampage.
IS sympathizers have killed more than 200 people in France in recent years - 130 of them in an coordinated attack on Paris revelers in 2015.
The latest attack Saturday occurred less than two months after a murderous extremist rampage in the southern town of Carcassonne, a haunt for tourists. There, a Frenchman of Moroccan origin - also on a watch list - killed three people with a gun, two in a supermarket, including an officer who exchanged himself for a hostage.
Azimov had been studying to become a nurse after failing to gain entrance to medical school, Molins said.
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