A car, whose owner is on an intelligence services watch list of people suspected of religious radicalization, was discovered near Notre-Dame cathedral in Paris on Saturday night with seven gas cylinders inside, police and judicial officials said on Wednesday.
The car owner was taken into custody but later released, one judicial official said. A couple, aged 34 and 29, were arrested at a highway rest area on Tuesday in southern France in connection with the incident and remained in custody, the official said.
The Peugeot 607, which had no registration plates, contained seven gas cylinders, one of them empty on the front passenger seat, two police officials said.
It was found with its hazard lights flashing, as if to attract attention, they said.
Interior Minister Bernard Cazeneuve said he was waiting for a report from investigators on what the possible motives were for the incident.
There was no detonating device present in the car, found on a Seine riverside stretch called the Quai de Montebello, meters from the Notre-Dame cathedral, one of Paris's many popular tourist attractions.
Documents with writing in Arabic were also found in the car.
More than 200 people have been killed in terror attacks over the past year and a half in France.
France remains on maximum alert after calls by the Islamic State group for followers to attack the country, which is bombing the militant group's bases in Iraq and Syria.
Florence Berthout, mayor of Paris's fifth arrondissement district, said the incident highlighted the need to beef up security and put more police on patrol in one of the world's most visited cities.
She said the vehicle was left in a zone where parking is strictly prohibited and that it had remained there for around two hours before it came to the attention of police.
"Police and army staffing must be stepped up," she told news TV channel BFM.
Thousands of extra police and soldiers have been deployed to patrol sensitive sites across France since 130 people were killed by Islamist gunmen and suicide bombers in multiple attacks on Paris last November 13.
A state of emergency declared at that time is still in place and gives police extra search and arrest powers but debate still rages over security levels, following a further attack on July 14 in which a man mowed into crowds in the city of Nice, killing 86.