At a ceremony at the police headquarters where they were stabbed to death in last Thursday's bloody rampage, a solemn Macron endured drizzle as he paid homage to the three police officers and one police administrator killed by their own colleague, a 45-year-old deaf technology administrator and Muslim convert.

"They had made the choice to wear the uniform, to devote their lives to protecting others. They died in service, at work," said Macron, who was also met privately with families of the victims.

French prosecutors are investigating the killings as a potential act of terrorism as it transpired the knifeman likely had links with members of an ultra-conservative Islamic movement.

"The whole nation (must) unite, mobilize, act... We will only win if our country gets up to fight against this underground Islamism that corrupts the children of France," he added.

He proposed establishing a "society of vigilance" to protect France, a country still reeling from numerous extremist attacks in recent years — but he warned the French against "suspicion that corrodes."

Though Interior Minister Christophe Castaner initially said there were "no warning signs" ahead of the attack, he has since acknowledged breaches in security over a failure to detect signals of the attacker's radicalization. The man had previously "justified" the deadly 2015 Islamic extremist attack on satirical newspaper Charlie Hebdo in front of his colleagues.  No written report was made at the time.

It took some 24 hours after the attack for authorities to say it was a potential act of terrorism, and the French government initially maintained there was nothing to suggest the armed attacker had any ties to extremist groups.

Earlier Tuesday, Castaner posthumously bestowed France's highest award, the Legion of Honor, on the four victims. The fifth fatality, the knifeman himself, was shot dead by a rookie officer who had completed police academy training six days before the attack.

Authorities said the attacker had worked for the Paris police force as a technology administrator in the intelligence unit since 2003 and didn't have a history of psychiatric problems.

Tuesday's ceremony came as justice officials said French investigators found a USB stick belonging to the killer containing information about his colleagues.

Officials did not immediately confirm several French media reports that the memory stick contained "jihadi propaganda."