French investigative sources said Wednesday the vehicle was discovered close to Paris' Notre Dame cathedral next to the River Seine on Sunday morning.

The car had its hazard lights on, no license plates and no one was inside.

Seven canisters filled with gas were found in the trunk and an empty one was in the car. But no detonators were discovered.

The security alert was raised by an employee of a nearby bar, who noticed the suspicious vehicle, the police sources said, on condition of anonymity.

A French police official said the couple that owned the car was taken into custody on Tuesday. Both of them were known to police.

French Interior Minister Bernard Cazeneuve said it still was not clear why the car was abandoned.

Notre Dame, a Gothic cathedral, is one of the city's key landmarks, attracting 13 million visitors each year.

France's state of emergency

France is on alert after a string of attacks by militants linked to the "Islamic State" (IS) group and threats against key buildings.

In July, close to 90 people were killed when a truck ploughed into a Bastille Day crowd in the southern resort of Nice, with IS 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.

Those incidents follow two deadly attacks in Paris last year, which left more than 140 people dead, also claimed by Islamist militant groups.

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.

News of the discovery of the vehicle broke hours before the release of a government-commissioned report on how the attacks have impacted France's lucrative tourism industry.

Gas canisters filled with nails were used in attacks on Paris in the 1990s by Algeria-linked extremists.

mm/se (AFP, AP)