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The fatal stabbing of four people in Paris yesterday is now being investigated as a possible act of terrorism.

A knife-wielding employee went on a rampage at the city’s central police headquarters, killing three officers and an administrative worker before being shot dead.

Michael Harpon, 45, who worked as an IT administrator, roamed two floors and critically wounded at least one other person.

Paris prosecutors said they decided to refer the investigation to the national anti-terror unit "in light of the evidence gathered by criminal investigators at this stage of the inquiry".

Paris Knife Attack: Four people killed at police headquarters 16 show all Paris Knife Attack: Four people killed at police headquarters 1/16 Police block the street after four people were killed in a knife attack at a police headquarters AFP via Getty Images 2/16 Emergency personel stand near an air ambulance helicopter on the Pont Marie near Paris prefecture de police (police headquarters) after three persons have been hurt in a knife attack o AFP via Getty Images 3/16 A policeman stands next to firefighter vehicles near the police headquarters AFP via Getty Images 4/16 French police secure the area in front of the Paris Police headquarters in Paris, France Reuters 5/16 Police officers patrol in a rubber boat on the Seine river after an incident at the police headquarters in Paris AP 6/16 Reuters 7/16 AP 8/16 AFP via Getty Images 9/16 AP 10/16 AFP via Getty Images 11/16 REUTERS 12/16 AFP via Getty Images 13/16 Reuters 14/16 AFP via Getty Images 15/16 AFP via Getty Images 16/16 AFP via Getty Images 1/16 Police block the street after four people were killed in a knife attack at a police headquarters AFP via Getty Images 2/16 Emergency personel stand near an air ambulance helicopter on the Pont Marie near Paris prefecture de police (police headquarters) after three persons have been hurt in a knife attack o AFP via Getty Images 3/16 A policeman stands next to firefighter vehicles near the police headquarters AFP via Getty Images 4/16 French police secure the area in front of the Paris Police headquarters in Paris, France Reuters 5/16 Police officers patrol in a rubber boat on the Seine river after an incident at the police headquarters in Paris AP 6/16 Reuters 7/16 AP 8/16 AFP via Getty Images 9/16 AP 10/16 AFP via Getty Images 11/16 REUTERS 12/16 AFP via Getty Images 13/16 Reuters 14/16 AFP via Getty Images 15/16 AFP via Getty Images 16/16 AFP via Getty Images

Investigators scoured the attacker's computer and mobile phone for clues to his motive and interviewed his wife, as well as witnesses.

Authorities said the 45-year-old converted to Islam 18 months ago.

His wife Iham, who was taken into police custody on Thursday but not charged, reportedly told police he heard voices on Wednesday night.

Officials said he did not have a history of psychiatric problems and had not presented any previous cause for concern.

Police union officials previously suggested the attacker may have been involved in a workplace dispute.

There were tensions between the knifeman and his supervisor, according to police union official Christophe Crepin.

"I do not think this is a terrorist act," Mr Crepin, who knew the attacker, told Franceinfo Radio.

Police union leader Jean-Marc Bailleul described it as a criminal act, telling BFMTV: "It was a moment of madness."

The attacker is said to have gone into the building at around 1pm and straight to his office where he began attacking colleagues with a knife.

He stabbed three people inside two offices and two women on a stairway, before he was shot dead by an officer inside the building's courtyard, according to French media.