PARIS (AP) — Investigators found nothing so far to suggest a civilian police employee who converted to Islam 18 months ago had been radicalized before a knife attack inside police headquarters that killed four people, the French government said Friday.

The assailant, a technology administrator in the intelligence unit, was shot dead by a rookie officer who had completed police academy training six days before the attack, Paris police chief Didier Lallement said.

Assigned to security duty at the large police compound across the street from Notre Dame Cathedral, the rookie ran into the courtyard to counter the attack and confronted the 45-year-old administrator, Lallement said.

The police force held a minute of silence in the same courtyard on Friday morning. Three police officers and an administrator were stabbed to death before the attacker was stopped. Another person received serious injuries.

"We've been hurt at the heart, but we are still standing," the police chief said.

Investigators were scouring the attacker's computer and cellphone for clues to his motive and interviewing his wife and witnesses, a police official said. He had worked for the Paris police force since 2003.

The wife told police her husband, who was deaf, had visions and made incoherent statements during the night before the attack, France Info and broadcaster BFM TV reported.

He converted to Islam 18 months ago, wasn't believed to have been radicalized by extremists, and didn't have a history of psychiatric problems, according to the official, who spoke on condition of anonymity to discuss the ongoing investigation.

Paris prosecutors opened a murder investigation on facts "which are not falling under terrorism" for now," French government spokeswoman Sibeth Ndiaye told broadcaster France Info.

However, Paris police chief Lallement said no theories are off the table and "we don't rule out any hypothesis at this stage."

Asked about the ceramic knife the employee brought into the building and used to stab people, police spokeswoman Laetitia Vallar told BFM TV that police officers and administrators can enter under an officer's supervision without undergoing security screening.

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Nicolas Vaux-Montagny contributed to the story.