PARIS (AFP) - French authorities released the parents of Paris knife attacker Khamzat Azimov from custody on Tuesday (May 15), while keeping his closest friend in detention, a judicial source said.

Azimov, a naturalised French citizen of Chechen origin who had lived in Strasbourg, was shot dead by police on Saturday night after stabbing a 29-year-old man to death in the Opera district of Paris and wounding four others in an assault claimed by the Islamic State group.

Azimov's parents were detained on Sunday morning. An apartment that his parents had rented in Paris was also searched.

The attacker's close friend, named as Abdoul Hakim A., a 20-year-old of Chechen origin from Strasbourg, was retained in custody until Thursday.

He was arrested by heavily armed, masked police on Sunday at his home in the eastern city.

A source close to the inquiry told AFP he had been under surveillance since marrying Ines Hamza, a woman from the Paris region who tried to leave for Syria in January 2017.

Police recovered seven cellphones during a search of his home but were unable to locate the main phone he used most often.

Always together

Saturday's knife rampage followed a series of jihadist assaults that have now claimed 246 lives in France since 2015, including the Paris attacks of that year and the truck-ramming attack targeting Bastille Day revellers in Nice in 2016.

A former student in Strasbourg told AFP that Azimov and Hakim A. were in the same class during their final year, "and were very good friends, both Chechen - they were always together, both in school and outside." Azimov's family later moved to Paris, where they were renting rooms in the 18th arrondissement.

Azimov had been on both of France's main watchlists for suspected radicals since 2016 - the so-called "S file" and a more targeted File for the Prevention of Terrorist Radicalisation (FSPRT), which focuses on people judged to be terror threats.

The government has come under fire over the fact that Azimov had been flagged as a suspected extremist, just the same as several others behind deadly attacks including the brothers who carried out the Charlie Hebdo gun massacre in 2015.

ISIS claim

Azimov became a French citizen in 2010 after his mother was naturalised.

Hundreds of fighters from Chechnya have joined Islamist militant groups in recent years, following two bloody separatist wars against Russia.

Amaq, the ISIS propaganda agency, released a video Sunday in which it claimed responsibility for the attack, with footage claiming to show Azimov pledging allegiance to the jihadist group.

In Paris, witnesses said Azimov walked along stabbing people and yelling "Allahu akbar" (God is greatest).

The seriously injured included a 34-year-old Chinese man who lives in Luxembourg and a 54-year-old woman. Two others were lightly wounded but all four are out of danger.

The 29-year-old man who was killed has been identified as Ronan, whose neighbours in Paris's 13th arrondissement posted a letter announcing his death in the hall of their building.

Erik Gouenard, a nearby restaurant owner, described him as "discreet, quiet but always smiling. It was like a smile was part of his DNA."