A man who rammed his car into a police van near Paris’ Champs Elysees avenue on Monday had pledged allegiance to Islamic State militants, a source close to the investigation said on Tuesday.

The source said the man, who died in the attack, had given a letter to his brother-in-law announcing his support for the group that controls parts of Syria and Iraq.

The man stored a cache of weapons at his home and held a gun permit despite being on a secret service list of people linked to radical Islam, police sources and French officials said Tuesday.

A judicial source said investigators were compiling an inventory of the arms and equipment found in the 31-year-old’s home. The man, who died in the attack, was also carrying in his car an assault rifle, two pistols, ammunition and two large gas canisters when he rammed a police convoy Monday.

Prime Minister Edouard Philippe said the individual first received a permit to possess a gun before he was flagged to intelligence agencies as a potential militant threat. At the time there was no reason to deny him the permit, Philippe said.

Philippe said it was “quite possible” the license was active at the time the attacker was on a security watchlist. Three sources close to the investigation confirmed it was.

“Nobody can be happy, and certainly not me, that someone who has been flagged to security agencies can continue to benefit from such an authorisation,” Philippe told BFM TV.

The man was placed on France’s so called ‘Fiche S’ watchlist after he was found to belong to a radical Islamist movement, two police sources said.

Individuals on the list are placed under surveillance though the intensity of that surveillance varies depending on the perceived threat the individual poses.

Philippe said draft legislation drawn up in May envisaged changes to allow officials who handle gun permits to check if individuals seeking licenses are on a watchlist.

But refusing permits in such cases had it drawbacks, he said. “If you revoke the authorisation of someone who is under surveillance, they’re going to know why.”

On Monday, witnesses saw the man being pulled from the car as thick yellow smoke poured out.

Police arrested four of his close relatives in a raid south of Paris late on Monday, a police source said. They included his father and brother.

France has been on high alert after a wave of militant Islamist attacks over the past two years, including most recently an attack on police outside the Notre Dame Cathedral and an Islamic State-claimed attack on police on the Champs Elysees in April.

In July last year, 86 people were killed when a truck plowed through a crowd in Nice, and similar incidents have occurred in other European cities.

Philippe said the government would be presenting a draft law soon to toughen counter-terrorism legislation.

“We need to find legal instruments that at once guarantee that we continue to live in a Fifth Republic which safeguards freedoms and ensure the security of French people,” Philippe said.