French prosecutors opened an investigation Friday that treats the fatal knife attack that a civilian employee carried out at Paris police headquarters as a potential act of terrorism.

The Paris prosecutors’ office said information from researching the background of the longtime employee who killed three police officers and an administrator led to the probe for multiple murders “in relation with a terrorist enterprise.”

The prosecutors’ office did not release further details.

Investigators scoured the 45-year-old attacker’s computer and cellphone Friday for clues to his motive, and also interviewed his wife and witnesses. A rookie police officer shot and killed him in a courtyard during Thursday’s attack.

A minute’s silence was held by police HQ in Paris on Friday in memory of the four staff members who died.

A crowd gathers outside police headquarters in Paris with French officials attending the moment of silence for the four victims. (@prefpolice/Twitter)

Authorities said the assailant, a technology administrator in the police intelligence unit, had worked for the Paris police force since 2003. 

Officials had said earlier Friday that the man didn’t have a history of psychiatric problems and investigators didn’t have evidence at that point indicating he had been radicalized by extremists.

A day prior to the killings, police across France went on a strike over concerns of increasing violence faced by officers.