A car packed with several gas cylinders was discovered close to central Paris’ Notre-Dame cathedral last Saturday and its owner, now in custody, is on an intelligence services watchlist of people suspected of religious radicalization, police officials said.
The car owner’s associate, known to police, was also arrested, sources close to the investigation said.
But Le Parisien, the capital’s daily, reported that six people are now being held.
The Peugeot 607, which had no registration plates, contained seven gas cylinders, one of them empty on the front passenger seat.
It was found with its hazard lights flashing, as if to attract attention, two police officials said on Wednesday. “We think he may have been trying to carry out a test-run,” one of the officials said.
No detonators were present in the car, which was found on a Seine riverside stretch called the Quai de Montebello, meters from the Notre-Dame cathedral.
Documents with writing in Arabic were also found in the car.
Notre Dame is one of Paris’ most visited landmarks attracting 13 million visitors each year.
More than 200 people have been killed in terror attacks over the past year-and-a-half in France.
In July, 86 people were killed when a truck ploughed into a Bastille Day crowd in the southern resort of Nice with ISIS saying it was driven by one of its followers.
Less than two weeks later, two young jihadists murdered a priest near the northern city of Rouen.
ISIS also claimed responsibility for the November 2015 attacks on Paris that killed 130 people and injured hundreds.
France remains on maximum alert after calls by ISIS for followers to carry out more attacks in the country, which is bombing the terrorist group’s bases in Iraq and Syria.