Tuesday's attacker had “never showed any sign of radicalisation,” he told RTL radio, but stressed that the investigation was at an early stage.
Castaner said the man, who was shot and injured by police, named as Farid Ikken “had at no moment showed signs of radicalisation.”
On Wednesday a source close to the investigation confirmed to AFP that the attacker had pledged allegiance to the Islamic State group in a video found at his apartment.
The 22-year-old police officer sustained minor neck injuries in the assault, which comes with France on high alert after jihadists killed seven people in London on Saturday.
The video below taken from surveillance cameras, shows the moment the attacker struck, before he was neutralized by a police officer.
The incident was over in a matter of a seconds.
Documents found on the attacker identified him as Farid Ikken a 40-year-old Algerian who was a doctoral student in information science at a university in the east of France.
The suspect later claimed to be a “soldier of the caliphate” of the Islamic State group, according to a source close to the investigation.
The policeman's colleague opened fire on the man, hitting him in the chest in panicked scenes around the Gothic cathedral that is one of France's most visited tourist attractions.
The attacker lay bleeding on the ground as police sealed off the area and searched for possible accomplices.
About an hour after the attack he was taken to hospital and police declared the situation to be under control.
Interior Minister Gerard Collomb said the man, who was also carrying kitchen knives, had shouted “this is for Syria” as he lunged at the officer.
Anti-terrorist prosecutors were put in charge of the investigation.
A witness told AFP he heard someone “shout very loudly”.
“Then there was a crowd surge and people panicked. I heard two shots and saw a man lying on the ground in a pool of blood,” he said.
On Tuesday evening a group of 15 heavily armed elite police officers searched student accommodation in the suburbs of Paris, where the suspect rented a studio, an AFP journalist said.
A tenant of the building described him as “very quiet”.
France is still under the state of emergency imposed after the November 2015 attacks in Paris, when Islamic State jihadists killed 130 people in a night of carnage at venues across the city.