French detectives are continuing to work to establish the motive of a state employee who killed four colleagues with a kitchen knife inside Paris’s police headquarters on Thursday.

The assailant, named in the media as Mickaël H, 45, was a computer scientist in the intelligence branch at police headquarters, and had worked for the service for 15 years. He was was stationed in one of the most important departments, which coordinated anti-terrorist intelligence-gathering in the capital.

Investigators are still attempting to determine why at lunchtime on Thursday, deep inside the vast police complex near Notre Dame in the centre of Paris, the man took a ceramic knife and went on a killing rampage that lasted only a few minutes.

First, in an office in his own department, he killed three police officers. Then he killed another staff member and injured two more in a stairway before heading towards the exit. A young intern police officer, who had worked in the building for only a few days, called for the man to drop the knife. When he did not, the officer shot him in the head, killing him.

Police on Friday continued to search the assailant’s home in a small town near Charles de Gaulle airport north of Paris. Data from his computers and phones was also being assessed. Within hours of the attack, investigators in balaclavas were seen removing computer equipment from the property.

Questions remain over what might have led the man to carry out the attack, whether it was personal or premeditated and whether he entered the building with a knife. There were no surviving witnesses from the room where he first attacked and killed three officers, so it was not clear what he may have said to them as he launched the attack.

The attacker was said to be a quiet and unassuming member of staff, and no change in his behaviour had been noted in the weeks before the attack. The interior minister, Christophe Castaner, said the assailant had “never shown any behavioural problems” and never aroused the “slightest reason for alarm”.

The incident is so far being investigated by a regular Paris police unit and being treated as a case of homicide, not terrorism. The investigation has so far not been referred to anti-terrorist police. Officials said there was no indication the attack was an act of terror, but the Paris police chief, Didier Lallement, told reporters on Friday no theory was being ruled out at this stage. He said the Paris police force “will be forever marked by this drama. We will not forget.”

The attacker’s widow told investigators that her husband, who had a severe hearing disability, displayed “unusual and agitated behaviour” the night before his crime, a source close to the investigation told AFP.

The assailant had converted to Islam 18 months ago, but initial searches at his home showed no sign of any radicalisation. He was not on any watchlist.

The government spokeswoman Sibeth Ndiaye urged caution. She said the possibility of a terror motive had not been ruled out but she added: “It is important to emphasise: you are not a terrorist because you are Muslim and converting to Islam is not an automatic sign of radicalisation.”

She told France Info radio: “The facts need to be looked at carefully.”

The attack came a day after thousands of officers marched in Paris to protest against low wages, long hours and an increasing suicide rate in their ranks.