PARIS (Reuters) - France’s anti-terror prosecutor has taken over the lead in investigating a knife attack in which an IT worker at police headquarters in Paris killed four co-workers before a police officer shot him dead.

Officials have not said there was a terror motive behind Thursday’s attack in the center of Paris near Notre-Dame cathedral, but the decision to hand the case to anti-terror prosecutors usually indicates a terrorism link is the focus of inquiries.

A judicial source close to the investigation said the anti-terror prosecutor took over the investigation following the interrogation of several of the attacker’s close associates, including his wife, and an examination of his cellphone.

The 45-year-old attacker, who had worked at the police headquarters for several years, had converted to Islam around 18 months ago, French media reported.

At lunchtime on Thursday he went on a rampage that spanned two floors of the headquarters building, killing three police officers and an administrative worker, and wounding at least one other, before being shot dead.

“Based on the information gathered at this stage in the investigation by investigators in the criminal unit, the national anti-terrorist prosecutor has this evening taken on the case,” a prosecutor’s office spokesman said.

The attack took place on the historic Ile de la Cite island in the River Seine. The area was locked down after the attack, with armed security personnel in camouflage gear patrolling a bridge over the river.

The attacker’s wife on Thursday was taken into police custody but not charged. During interviews with police, she described how her husband had heard voices on Wednesday night, according to the source close to the investigation.

A second source informed about the investigation said the attacker was believed to have had an “attack of insanity” that led him to carry out the attack.

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The killer, identified by sources close to the investigation as Mickael H., had previously given his employers and co-workers no cause for concern about his behavior, officials said.

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MULTIPLE CASUALTIES

In the past four years, the French capital has been rocked by major attacks resulting in mass casualties.

Co-ordinated bombings and shootings by Islamist militants in November 2015 at the Bataclan theater and other locations around Paris killed 130 people in the deadliest attacks in France since World War Two.

The knife attack unfolded just after midday on Thursday when the attacker, on the first floor of the police headquarters, used a kitchen knife to stab three policemen before taking the stairs down to the ground floor of the building.

On the staircase he came across two women staff, according to Le Parisien newspaper, which cited an internal police report on the incident. He stabbed both of them, fatally wounding one.

He then moved out into the courtyard of the police headquarters. There, according to the Parisien account, a policeman responsible for security at the building issued several warnings, then fired several shots at the attacker, including to his head, killing him.

The officer used a Heckler & Koch G-36 assault rifle, the newspaper reported.

The police officer who halted the attack had recently completed his training and had been in the job for only six days, Paris police chief Didier Lallement told reporters.