The source told Reuters that another person close to the suspected attacker and believed to have been a former roommate in a hotel north of Paris had been arrested, following a series of other arrests on Friday evening.

On Saturday morning, seven people remained in custody including the suspected attacker. A police source said the suspected attacker was cooperating with the police.

The attack took place in front of a building where extremist militants gunned down employees of satirical magazine Charlie Hebdo in 2015.

It coincided with the start this month of the trial of 14 alleged accomplices in the 2015 Charlie Hebdo attack. The gunmen behind that attack killed 12 people.

Police quickly detained the man suspected of carrying out the attack next to the steps of an opera house about 500 meters away.

The suspected attacker was from Pakistan and arrived in France three years ago as an unaccompanied minor, Interior Minister Gerald Darmanin said.

A second suspect was detained moments after the attack and prosecutors were trying to establish his relation to the attacker. He was released free of charge, the source said.

Charlie Hebdo vacated its offices after the 2015 attack and is now in a secret location. The building is now used by a television production company.

Two of the production company’s staff, a man and a woman, were in the street having a cigarette break when they were attacked, according to prosecutors and a colleague of the victims.

Al-Qaeda, the militant group that claimed responsibility for the 2015 attack, threatened to attack Charlie Hebdo again after it republished the cartoons of Prophet Muhammad (PBUH).

France has experienced a wave of attacks by extremist militants in the past few years.

Bombings and shootings in November 2015 at the Bataclan theatre and sites around Paris killed 130 people, and in July 2016 an extremist militant drove a truck through a crowd celebrating Bastille Day in Nice, killing 86.