At least two people, including one on the French intelligence services’ watchlist, were taken into custody after police found several gas canisters in an abandoned vehicle next to Notre Dame Cathedral in Paris, officials said, as cited by French media.

The car, a Peugeot 607, was found parked next to one of the Paris’ most famous attractions on Sunday morning with at least seven gas cylinders inside. One empty canister was sitting on the passenger seat, but no detonators were found during the investigation, sources told Le Figaro.

Police had initially arrested at least six people in connection with the case, but iTele later reported that four have been released and two suspects remain in custody, of whom one is a woman on the “Fiche S” list, an indicator used by France to flag people considered to be a threat to national security. Those on that list are not arrested, but are thoroughly monitored by the authorities.

Police officer stabbed in Paris suburb, attacker shot dead https://t.co/2k7krBu7aL — RT (@RT_com) September 3, 2016



Le Figaro added that the two being detained were a 29-year-old woman and 34-year-old man, and police told AFP sources that the pair was previously known to law enforcement, and one of the suspects is believed to be the owner of the car.

“The car was parked for almost two hours, according to my information,” in an area where parking “is forbidden,” Florence Berthout, the head of the neighborhood where the car was found, told BFMTV.

The car was found with its hazard lights flashing as if to attract attention, police said, and documents found inside the vehicle were in Arabic.

“We think he may have been trying to carry out a test-run,” one of the officials said, as cited by Reuters.

#ISIS planned more attacks across Europe after Paris massacre of November 2015 https://t.co/nRZcsEscJIpic.twitter.com/tfvL0ffvNe — RT (@RT_com) September 6, 2016

France has been on high alert since January of 2015, when it was hit by a series of Islamic State (IS, formerly ISIS/ISIL)-linked terrorist attacks.

The biggest loss of life took place in November of 2015, when at least 130 people were killed in Paris. A tragedy in Nice on July 14 of this year killed at least 84 people when a truck driven by an IS sympathizer plowed through crowds during Bastille Day celebrations.

France was also shaken by the murder Father Jacques Hamel in northern Saint-Etienne-du-Rouvray in July, when the 85-year-old priest had his throat cut by two purported IS militants.