Following evidence gathered by criminal investigators, anti-terror police were investigating into the prefecture's assault that left four victims on Thursday, BFMTV news channel reported.

A probe into attempted murder of public agents in relation with terrorist enterprise was opened following examination of the attacker's cellphone and the questioning of his wife, it added.

Identified as Michael Harpon, the 45-year-old man attacked with a knife four people inside the building in center Paris. He killed three police officers and a female administrative assistant.

He also wounded another person before being shot dead by an officer.

He joined police headquarters in 2003 and had never displayed any behavioral disorder or showed any signs for concern, officials said.
The wife of the assailant, who converted to Islam 18 months ago, told investigators her husband showed "unusual and agitated behavior" the night before his crime, according to local reports.

Earlier, Paris police chief Didier Lallement said no hypothesis was ruled out on the probe into the attacker's motive, without elaborating.

But, the move to hand the investigation to anti-terrorism office means that authorities had established possible terrorist-link and believed the attack was deliberate.