The man believed to have attacked and wounded two people with a meat cleaver on Friday has told the police he was targeting weekly satirical magazine Charlie Hebdo, a police source told Reuters.
The incident took place in front of a building where Islamist terrorists killed 12 people in 2015 because of the republication of cartoons depicting Prophet Mohammad.
Earlier a suspected accomplice in Friday's attack was released, a judicial source said.
The source said a former hotel roommate of the suspected attacker had been detained though, following a series of other arrests on Friday evening.
The arrests meant that seven people remained in custody on Saturday morning, including the suspected attacker who is said to be cooperating with the police.
Friday's attack outside the former offices of Charlie Hebdo in Paris is being treated as suspected terrorism.
Suspect previously arrested but not on police radar
The suspected assailant had been arrested a month ago for carrying a screwdriver but was not on police radar for Islamic radicalisation, France's interior minister Gerald Darmanin said.
He said the suspect had arrived in France three years ago as an unaccompanied minor, apparently from Pakistan, but his identity was still being verified.
"Manifestly it's an act of Islamist terrorism," Mr Darmanin told public broadcaster France-2.
“Obviously, there is little doubt. It's a new bloody attack against our country, against journalists, against this society.”
The victims were two workers from a documentary production company who had stepped outside for a smoke break.
The suspects’ identities have not been released and it is unclear what exactly prompted the attack.
An investigation was opened into “attempted murder in relation with a terrorist enterprise,” according to an official at the terrorism prosecutor’s office.
French prime minister Jean Castex said the lives of the two injured workers were not in danger.
He noted the "symbolic site" of the attack "at the very moment where the trial into the atrocious acts against Charlie Hebdo is under way."
He promised the government’s “unfailing attachment to freedom of the press, and its determination to fight terrorism.”
Many of France's most celebrated cartoonists died at the former headquarters of Charlie Hebdo when two brothers, Said and Cherif Kouachi, opened fire on them.
The 14 people on trial are suspected of helping the Kouachi brothers carry out the attack.
Defiant as ever, the magazine republished cartoons of the Prophet Mohammed at the start of the trial.
Staff, who now work at a secret location, received death threats after it hit the stands.
While the edition sold out in France, it provoked condemnation from several Muslim countries and Al Qaeda militants reportedly threatened a repeat of the 2015 massacre of its staff.
More than 100 French news outlets called on people to support the magazine amid the threats.
Police cordoned off the area, including the former Charlie Hebdo offices, after a suspect package was noticed nearby, but the package was found to be harmless and no explosives were found, according a police official.
Audible gasps were heard at the terrorism trial currently under way as news of the attack filtered through.