Two people are in custody after a car parked near Notre Dame Cathedral in Paris was found with seven gas cylinders inside, and its licence plates removed.
An employee of a nearby bar flagged the car discovered on Sunday morning along the Seine River with its hazard lights on, according to two French officials.
No one was inside, but police found six canisters filled with gas in the trunk and an empty canister in the car.
The officials said the car's owner and another person were taken into custody.
French Interior Minister Bernard Cazeneuve said it still was not clear why the car was abandoned.
No detonator or materials that could have been used to ignite the canisters were located in the car, the officials said.
France is on alert after a deadly string of Islamic State attacks and threats against landmarks. News of the discovery 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.