Police in Paris
Police in ParisReuters

France's interior minister acknowledged on Sunday that officials should have kept a closer eye on the Paris police employee who stabbed four colleagues to death last week, after investigators found evidence he had supported an extreme version of Islam.

"Obviously, there were failings," Christophe Castaner told TF1 television, as quoted by AFP.

Castaner had come under fire after initially claiming that Mickael Harpon, a 45-year-old computer expert at the Paris police headquarters, had never given the "slightest reason for alarm" ahead of Thursday's attack.

Investigators on Saturday revealed that Harpon had in fact been in contact with adherents of Salafism, the ultra-conservative branch of Sunni Islam.

On Sunday, Castaner said Harpon had caused alarm among his colleagues as far back as 2015, when he defended the massacre of 12 people at the Charlie Hebdo newspaper by two brothers who vowed allegiance to Al-Qaeda.

But even though a police official charged with investigating suspected radicalization among the force questioned the colleagues, none of them wanted to file an official complaint.

"Apparently they decided not to make a report," Castaner said. "The failure occurred at this moment.”

"There was nothing in his personnel file that indicated he might be radicalized... If there had been a sign, maybe we could have avoided this," he added.

Castaner will face questioning by parliament's intelligence commission Tuesday over the attack, its president Christian Cambon said Sunday.

"We're going to try to find out what these failings were," Cambon told AFP.

On Friday, French detectives referred the investigation into the knife rampage to anti-terrorist prosecutors.

The prosecutor said on Saturday that Harpon exchanged 33 text messages with his wife shortly before the attack.

The messages exclusively concerned religion, and the attacker ended the conversation with "Allahu Akbar" and told her to "follow our beloved prophet Mohammed and meditate on the Koran", according to the prosecutor.

France has been hit by a wave of terrorist attacks in recent years, beginning with the 2015 attack on the offices of the Charlie Hebdo satirical magazine which was followed by the attack on the Hyper Cacher kosher supermarket in which four people were murdered.

Since those attacks, France has been hit by a number of attacks claimed by the Islamic State (ISIS), the biggest one being the attack in November of 2015 in which 129 people were murdered.

The country has been under a heightened alert in recent years in the wake of the attacks.