French police have arrested a second couple in connection with a car found carrying seven gas cylinders near Notre-Dame cathedral in Paris, a judicial official said.
Police picked up the second couple on Wednesday evening about 110 kilometres south of Paris, a police source said.
A first couple, aged 34 and 29, were arrested on a motorway in southern France also in connection with the incident on Saturday and remain in custody.
They were known to the security services for links to radical Islamists.
There was no detonating device present in the car, found on a Seine riverside stretch called the Quai de Montebello, metres from Notre-Dame, one of Paris's most popular attractions.
Documents with writing in Arabic were also found in the car.
The car's owner, also known to authorities for spreading Islamist ideas, was also taken in for questioning but released on Tuesday evening.
Police are also looking for the owner's daughter, whom he described to investigators as radicalised.
'Boot open and gas cylinders on the ground'
A bar employee working near Notre Dame raised the alert on Sunday after noticing a gas cylinder on the back seat of the car, a police source said.
That cylinder was found to be empty but five full cylinders were found in the boot of the car.
Photographs of the metallic silver-coloured car after it was discovered showed its boot open and the gas canisters placed on the ground in a quiet side street opposite the cathedral.
Notre Dame, renowned for its flying buttresses, stained glass windows and gargoyles, is one of Paris's most popular landmarks, attracting 13 million visitors each year.
Speaking on Wednesday, Interior Minister Bernard Cazeneuve said the intentions of those arrested were as yet unknown.
Meanwhile, President Francois Hollande was due on Thursday to give a speech about democracy confronted by terrorism following a torrid summer in which 86 people were killed when a truck ploughed into a Bastille Day crowd in the southern resort of Nice.
IS said the truck was driven by one of its followers.
Less than two weeks later, two young jihadists murdered a priest near the northern city of Rouen.
Back in May, the head of France's DGSI domestic intelligence service, Patrick Calvar, warned in May of a "new form of attack" in which explosive devices would be left near sites that attract large crowds.
French security services are particularly worried about the danger posed by extremists returning from Syria after fighting with IS forces, with 700 French nationals still in the country, according to France's top prosecutor
More than 200 people have been killed in attacks by militant Islamists in France over the past 18 months.