Investigators are searching for possible motives for the knife attack at Paris police headquarters as the French government says there is nothing to suggest the police employee who stabbed four colleagues to death was radicalised.

The assailant, a 45-year-old technology administrator in the intelligence unit of the Paris police, was shot dead after yesterday's attack.

Government spokeswoman Sibeth Ndiaye said prosecutors have decided to open a murder investigation on facts "which are not falling under terrorism at this stage of the investigation".

Investigators searching for the motive of a 45-year-old police employee who killed four people in the Paris police headquarters say there is no evidence he was radicalised. (Getty)

Investigators were scouring the attacker's computer and mobile phone for any indications of a motive and were interviewing his wife and witnesses, a police official said.

The man, who converted to Islam 18 months ago, was not believed to have been radicalised and had no history of psychiatric problems, according to the official.

Interior minister Christophe Castaner said yesterday that the man had never shown any behavioural problems.

The assailant, a 45-year-old technology administrator in the intelligence unit of the Paris police, was shot dead after yesterday's attack. (AP/AAP)

News broadcaster France Info and BFM TV said the attacker's wife told police that her husband, who was deaf, had visions and made incoherent statements on the night before the attack.

Noam Anouar, of the Vigi police union, said the assailant had security clearance to work in the police intelligence unit.

Asked about the ceramic knife the assailant took into the building, Laetitia Vallar, spokeswoman at the Paris police, said on BFM TV that officers and administrators could enter the building with their badges under an officer's supervision.

A murder investigation has been opened into the incident. (AP/AAP)

They were not submitted to bag searches and security screening like the public.

She added that psychological assistance was being offered to officers and employees.

The knife attack left three officers and an administrator dead. Another person was seriously injured.

Police held a minute of silence in the headquarters' main courtyard this morning.