French counter-terrorism police have taken over the inquiry into an attack by 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 police for 15 years. He had full security clearance in an office that coordinated counter-terrorist intelligence gathering in the capital.

The case was handed to counter-terrorism police after analysis of the assailant’s phone and messages between him and his wife was carried out. Police sources told Agence-France Presse that preliminary inquiries suggested he could have become radicalised.

The attacker worked in a section of the police service dedicated to collecting information on jihadist radicalisation and had security clearance, raising issues about the force’s own system of internal security checks.

Counter-terrorist investigators were working to find out what led him to launch a knife attack on police officers he worked with, whether it was premeditated and whether he entered the building with his own knife.

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 launched a killing spree that lasted only a few minutes. First, in an office in his own department, he killed three police intelligence officers. Then he took a stairway, killing another staff member and injuring two more 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 young officer shot him in the head, killing him.

There were no surviving witnesses from the room where he first attacked and killed three officers, so it is not clear what he may have said to them before the attack.

Initially the killings had been treated as homicides and investigated by regular police. As searches and checks continued at the assailant’s home, the counter-terrorism force was called in and police officials said no motive was being ruled out.

Police on Friday continued to search the attacker’s home in a small town near Charles de Gaulle airport north of the city. 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.

The assailant, who was French and born on the island of Martinique, had converted to Islam 18 months ago, but initial searches at his home were reported to have shown no sign of any radicalisation. He had not been on any watchlist.

The government spokeswoman, Sibeth Ndiaye, said on Friday morning that the possibility of a terror motive had not been ruled out. But she said: “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 interior minister, Christophe Castaner, said the assailant had “never shown any behavioural problems” and never aroused the “slightest reason for alarm”. 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.

Paris’s police chief, Didier Lallement, said on Friday that the force would 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. She was questioned all day on Friday.

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.