French detectives on Friday referred the investigation into a knife rampage by a staffer at Paris police headquarters that left four colleagues dead to anti-terrorist prosecutors, sources said.
Three police officers and an administrative worker -- three men and a woman -- died in the frenzied 30-minute attack on Thursday at the police headquarters, a stone's throw from the Notre-Dame cathedral in the historic heart of Paris.
The assailant, a 45-year-old computer expert, was eventually shot dead by police.
Two other people were injured in the Thursday lunchtime stabbing spree that sent shock waves through an embattled French police force already complaining of low morale.
Sources at the Paris prosecutor's office, which had been handling the inquiry, said on Friday that the case had been passed to the anti-terrorist prosecutor's office (PNAT).
The attacker, named as Mickael H, was born on the French overseas territory of Martinique in the Caribbean. He was partially deaf.
Preliminary enquiries suggest that the attacker, a convert to Islam, could have become radicalised, said sources, who added that he had worked in a section of the police service dedicated to collecting information on jihadist 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, sources said.
Investigators were examining text messages between him and his wife, whose detention was extended on Friday.
- A 'quiet person' -
A search of the couple's house in a low-income Paris suburb near Charles de Gaulle airport yielded no evidence that the man, who became a Muslim about 18 months ago, had been motivated by radical religious ideology, one source added.
Computer equipment seized in the search was still being examined. AFP correspondents saw investigators -- faces concealed by balaclavas -- lead out from the house late Thursday a man and a woman who hid behind hoods.
One motive investigative sources mooted was a personal conflict at work.
"He complained to me that his career had not evolved, due to his handicap," said Abdelaziz, the deputy head of a local Muslim organisation who attended the Gonesse mosque with the attacker.
Locals said the man had two children aged three and nine.
"He was a very quiet person. I used to see him going to the mosque but he practised (his religion) in a normal way," a neighbour, who asked not to be named, told AFP.
Interior Minister Christophe Castaner said Thursday the assailant had "never shown any behavioural problems" and never aroused the "slightest reason for alarm".
The man used a kitchen knife to stab two policemen and a male administrative colleague in two offices on the first floor of the building before attacking two women on the staircase on his way to the courtyard outside.
One was a police officer who died at the scene. The other, an administrative worker, was seriously injured.
The assailant wounded another policeman in the courtyard before he was shot in the head by a colleague, sources told AFP.
- 'One of us' -
Tension has been growing in police ranks, stretched to the limit after months of weekly "yellow vest" anti-government demonstrations and years of high alert following a string of terror attacks in France.
Hundreds of police and headquarters staff observed a minute of silence in the courtyard of the building Friday in memory of their fallen comrades.
"This tragedy is all the more awful as it took place inside the police headquarters and it came from one of us," Paris's top policeman Didier Lallement said.
The killings came a day after thousands of police officers marched in Paris for better working conditions, a rare protest against the backdrop of a spike in police suicides -- 52 so far this year.
The police have also been a recurring target of jihadist groups, such as Islamic State, behind a wave of attacks since 2015 -- from large synchronised assaults to isolated knife and gun attacks.
In June, a parliamentary report on radicalisation within the public services spoke of 30 suspected cases out of the 150,000 police officers and 130,000 gendarmes in France.