The probe into a knife attack at the Paris police headquarters, which left four people dead, has been passed to France's terrorism prosecutors, it was announced on Friday.
A member of staff killed three policemen and a administrative worker on Thursday. He was later shot dead inside the building that sits on the Ile de la Cite in central Paris.
The man, a 45-year-old, was employed at the police station since 2003. He had two children and was partially deaf.
A background search led to the investigation for murders committed "in relation with a terrorist enterprise" and "criminal association with terrorists," the Paris prosecutors' office said.
David Le Bars, head of the Union of National Police Commissioners, told French broadcaster BFM TV it came from easily accessible sources found in a search of the attacker's home.
"We knew that searching through his computer histories, the websites visited, his relations, we would quickly have some information," Le Bars said.
He called the suspicion the slayings of three police officers and an administrator resulted from an extremist plot "a cataclysm" since the attacker worked for the police department. A fifth person was seriously injured.
Preliminary enquiries suggest that the attacker, a convert to Islam, could have become radicalised, sources told AFP, who added that he had worked in a section of the police service dedicated to collecting information on extremist radicalisation.
He had worked for the police since 2003 without ever arousing suspicion. But his wife, who is being questioned by police, claimed he had behaved in an "unusual and agitated" manner the night before the attack, according to French media reports.
She also said that he had visions and made incoherent statements.
Investigators were examining text messages between him and his wife, whose detention was extended on Friday.
