French prosecutors have opened an investigation that treats a fatal knife attack at Paris police headquarters as a potential act of terrorism.

Three hours after Paris police chief Didier Lallement made no mention of possible extremist links after the Thursday stabbing deaths of four people, prosecutors issued a statement late on Friday afternoon saying the 45-year-old civilian IT worker had a “criminal association with terrorists”.

He had worked as a technology administrator and had been employed with the Paris police force since 2003. He reportedly didn’t have a history of psychiatric problems, and converted to Islam 18 months ago.

His wife, who was taken into custody hours later, told police her husband, who was deaf, had visions and made incoherent statements during the night before the attack, BFM TV and France Info reported.

The long-time police employee stabbed four colleagues to death on Thursday before he was shot and killed by police inside the police compound, located in the same precinct as the Notre Dame Cathedral.

The Paris prosecutors’ office said in a background search led to the investigation for murders committed “in relation with a terrorist enterprise” and “criminal association with terrorists”.

Union of National Police Commissioners head David Le Bars 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,” Mr Le Bars said.

The office announced the decision in a two-line statement on Friday and gave no details about the evidence that persuaded prosecutors a terror investigation was warranted.

Mr Le Bars 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.

Investigators scoured the man’s computer and mobile phone on Friday local time for clues to his motive.

The attack came a day after thousands of officers marched in Paris to protest low wages, long hours and increasing suicides in their ranks.

The area around the police headquarters was sealed off and the nearest metro station was shut for security reasons, the transport authority said.

Extremists have repeatedly targeted French police in France in recent years. In 2017, a gunman opened fire on the Champs-Elysees boulevard, killing one officer before he was shot to death.

In 2016, an attack inspired by the Islamic State group killed a police officer and his companion, an administrator, at their home in front of their child.

-with AAP