Paris - Questions arose on Tuesday over how a known radical Islamist who rammed a car laden with weapons and gas canisters into a police van on Paris's Champs-Elysees was able to hold a firearms licence.
Adam Djaziri, a 31-year-old who had been on a watch list for radical Islamists since 2015, was killed on Monday as his car smashed into the police van on the French capital's most famous avenue.
Two handguns and a Kalashnikov-style assault rifle were found in the car, while a weapons stash was discovered at the home of the attacker.
A source close to the inquiry said a letter had been found in which Djaziri claimed allegiance to Islamic State group leader Abu Bakr al-Baghdadi.
Djaziri's father, who has since been detained, said that his son practised shooting as a sport and a source close to the probe said he had nine registered weapons including pistols and an assault rifle.
In the letter, Djaziri boasted that he was playing a "double game" by amassing weapons for an attack by posing as a shooting enthusiast, the source said.
The head of the French Shooting Federation said police officers had visited Djaziri's shooting club to enquire about him...implying that his keen interest in guns had raised suspicions.
The attempted attack comes as France remains under a state of emergency after a wave of jihadist assaults that have left more than 230 people dead since 2015.
As the month-old government of President Emmanuel Macron prepares to unveil a tougher new anti-terrorism law, Prime Minister Edouard Philippe expressed dismay that Djaziri was able to have a gun permit.
"What I know at this stage is that the first weapons permit was given before this individual was flagged" he said in an interview, but added that "no one can be satisfied... and certainly not me" that Djaziri was able to possess dangerous weapons after being put on a watch list.
Djaziri's ex-wife, brother and sister-in-law were detained late on Monday after police questioned them at the family home in Plessis-Pate, south of Paris.
Burn marks were found on Djaziri's body but it was not yet clear how he died. There were no other casualties from the attempted attack.
Since the large-scale Paris attacks in November 2015 and last year's Nice truck attack, France has suffered a string of smaller assaults targeting security forces.
Djaziri died just a short distance from the spot on the Champs-Elysees where a jihadist shot dead a police officer two months ago.
Earlier this month an Algerian man attacked a policeman with a hammer outside Notre Dame cathedral, another key tourist draw, while troops shot dead a man at the capital's Orly airport in March after he attacked a soldier on patrol.
In February, a man armed with two machetes assaulted four soldiers patrolling outside the Louvre museum.