French security officials questioned family and friends of Khamzat Azimov on Monday as the terror group ISIS claimed responsibility for the attack.
Mr Azimov, a naturalised French citizen, was shot dead by police on Saturday night after stabbing a 29-year-old man to death in the busy Opera district of central Paris and wounding four others.
The dead man was the 246th victim in extremist attacks since 2015 in France. Although seven per cent of all French people who have gone to fight in Syria are Chechens, it is the first atrocity perpetrated by someone from the community in the wave of crime.
The detained friend was said by locals to be the "individual closest" to Mr Azimov, whose parents have also been taken into custody.
The young man was arrested by heavily armed, masked police on Sunday at his home in Strasbourg.
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He was escorted out in handcuffs with his face covered, wearing a t-shirt that said "Defend Grozny" - the capital of Russia's North Caucasus region of Chechnya - with a picture of a machine gun.
The ISIS propaganda agency Amaq posted the video online using Telegram, featuring a young man wearing a hood with only his eyes exposed and the lower part of his face covered by a black cloth.
Speaking in French, he vows allegiance to ISIS leader Abu Bakr Al Baghdadi.
The attack in the Right Bank on Saturday night by a knifeman, later shot dead by police, left one person dead and four wounded.
"The author of this knife attack in Paris is a soldier of the Islamic State and the operation had been carried out in retaliation against the states in the coalition," said a "security source" at Amaq, referring to the international forces, including France, that are fighting militants in Syria and Iraq.
The attacker was a French citizen who was born in Chechnya and had been on two watchlists for suspected extremism.
His name was on the "S file" surveillance system and a more targeted "File for the Prevention of Terrorist Radicalisation" (FSPRT), which focuses on people judged to be terror threats.
In addition to Mr Azimov, who had been flagged as a suspected extremist, several others behind deadly attacks including the brothers who carried out the Charlie Hebdo gun massacre in 2015.
French media reported the victim was a 29-year-old man identified only by his first name, Ronan, who was living in the 13th district of Paris. One of his neighbours told reporters he was a "very smiling" man, with a "great generosity".