The French civilian police worker who murdered four colleagues with a knife has been linked to a radical Islamist terrorist organisation.

Michael Harpon, 45, was shot dead following the rampage inside the historic Paris Police Prefecture, next door to Notre Dame Cathedral, on Thursday afternoon.

His wife, Iham, was arrested soon afterwards and told detectives that Harpon had 'heard voices' and was 'incoherent and suffered a dementia attack' the night before.

The Paris knifeman who killed in the courtyard of police headquarters after killing four officers with a ceramic knife yesterday has been linked to a radical Islamist terrorist organisation

Now, following a raid on the couple's flat in the Paris suburb on Gonesse, anti-terrorist prosecutors in the French capital have opened an enquiry for 'assassination of a public servant in relation to a terrorist enterprise, attempted assassination of a public servant in relation to a terrorist enterprise, and conspiracy in terrorism'.

Computer records and telephone intercepts have linked Harpon to a 'radical Islamist group' such as ISIS of Al-Qaeda, said a source close to the case.

Iham Harpon remains in custody, and is also believed to have offered evidence against her husband.

In the heart of Paris: Police cordon off a road near the site of the massacre, with Notre Dame cathedral - currently closed after the fire in April - seen in the background

High security: Armed personnel block a bridge over the River Seine near the police compound where three men and one woman were killed yesterday

The couple were both registered as profoundly deaf, and have two children aged nine and three.

Iham is now communicating with officers through sign language, making her interview 'very complicated,' said the source.

Harpon, who originally came from the French overseas territory of Martinique, stabbed a senior officer and three guardians of the peace to death.

They have been identified as General Damien E., 50, Brice L., 38, Anthony L., also 38, and Aurelia T., 39.

The attack took place at the Paris police headquarters in the historic centre of the city, near the fire-ravaged Notre Dame cathedral

Comfort: People console each other on their way out of Paris police headquarters after the force's worst loss of life in a single day since World War II

Investigators now believe there was a terrorist motivation for the slaughter, which caused the worst loss of French police lives in a single day since World War II.

Harpon had converted to Islam 18 months ago, and his wife is also a Muslim who comes from an Arab background.

The nature of the attack corresponded with similar ones carried out by terrorists affiliated to ISIS and Al-Qaeda.

It is thought that the knifeman shouted 'Allahu Akbar' at his flat the night before he slaughtered four of his colleagues, according to a policeman who lived in the same building as Harpon.

'Many of those involved in similar attacks on the police have been Muslim converts influenced by radical terrorist groups,' said the source.

Paris police headquarters shared a photo this morning of hundreds gathered for a moment of silence in memory of their colleagues who were killed yesterday

Paramedics walk by fire engines and police vehicles near Paris police headquarters where the attack took place on Thursday afternoon

French police secure the area in front of the Paris police headquarters after the knife rampage

Colleagues described Harpon as a previously trusted employee who had full security clearance to work in the Prefecture's IT department.

The security crisis led to President Emmanuel Macron visiting the building in person, along with his Prime Minister, Edouard Philippe, and Interior Minister Christophe Castaner.

Harpon's ceramic knife would not have activated metal detectors, and he would not have been searched, they head.

Serious concerns have been expressed this year about the psychological frailty of police staff in France.

A brigadier who shot himself in the head with his own service pistol in front of shocked colleagues last night became the 50th French police officer to commit suicide this year.

Police are today examining whether Harpon, described as a 'very quiet person' by neighbours, held a grudge against his colleagues

Police officers patrol outside the police headquarters today after the frenzied knife attack yesterday

Emergency: A series of police and fire vehicles on the scene near Paris police headquarters where a civilian police worker killed four people on Thursday afternoon

The 45-year-old, who has not been named, died in the police station at Louvroil, close to the Belgium border on September 19.

President Macron had acknowledged that police officers have come under increasingly pressure because of growing security threats.

These include weekly demonstrations by the anti-government Gilets Jaunes – or Yellow Vests – movement, which frequently deteriorate into riots.