Anti-terror investigators have taken over the inquiry into the killing of four police employees by a colleague in Paris on Thursday.
A 45-year-old IT specialist killed three police officers and an administrative worker in a knife rampage at Paris police headquarters.
A motive is unclear, with a police union initially suggesting a workplace dispute was to blame.
But investigators say new information justifies a terror probe.
The attacker, Mickaël Harpon, was shot dead by a junior police officer after he attacked colleagues in several offices at the central police building in Paris.
He converted to Islam 18 months ago, according to reports, and had recently stopped talking to female colleagues in the office. But a government spokeswoman said on Friday morning that there was no indication he had been radicalised before the attack.
What do we know about the motive?
Police investigators initially said there was no indication the attack was an act of terror but Paris police chief Didier Lallement later told reporters no theory was being ruled out.
Until now, the case had been handled by the city's public prosecutor.
This changed on Friday evening when an official from the prosecutor's office said that national anti-terror prosecutors had taken over the investigation "in the light of the evidence gathered by criminal investigators".
Details of the new information uncovered have not been revealed.
But according to Reuters, quoting judicial sources, the move came about after several people close to the attacker, including his wife, were interrogated and his mobile phone analysed.
Police earlier seized computer equipment during a search of Harpon's home in the north of the city, according to media reports.
The killer's wife on Friday reportedly told police that he had been incoherent and had heard voices the night before the attack.
The woman, who is being detained but has not been charged, told police her husband had had a disagreement with his bosses.
Police union officials also described tensions between Harpon and his supervisor.
Harpon, who was born in the French overseas territory of Martinique in the Caribbean, worked in the force's intelligence unit. He also had a hearing and a speech disability.
The employee entered the building around lunchtime on Thursday, and proceeded to attack his colleagues with a kitchen knife.
He stabbed three people inside two offices and two women on a stairway, before he was shot dead by an officer inside the building's courtyard.
Three men and one woman were killed. A fifth person was seriously injured.
The police building is near major tourist sites including Notre-Dame cathedral.
Earlier on Friday, a minute's silence was held outside the headquarters in memory of the four staff who died.
Rassemblés pour un moment de recueillement et d'émotion intense, en présence du Ministre de l'Intérieur, du Secrétaire d’État et du préfet de Police, les agents de la préfecture de Police ont observé une minute de silence en mémoire des 4 fonctionnaires décédés. pic.twitter.com/v8XS7N1Gpf— Préfecture de Police (@prefpolice) October 4, 2019
Police shared a photo of the gathering on Twitter, calling it a "moment of contemplation" that was "deeply moving".
The police officer who shot dead the attacker had only been in his job for six days, according to Mr Lallement.