(AFP) – French police were holding two suspects Wednesday after finding several gas cylinders in a car near Paris’s Notre Dame cathedral, sources close to the investigation said.

The Peugeot 607 was discovered abandoned at the weekend with its hazard lights flashing, the sources said.

The owner of the car and another person, who are both known to security services, were arrested on Tuesday and anti-terror investigators have launched a probe.

France remains on high alert for terror attacks. In November, suicide bombers and gunmen killed 130 people in Paris in a series of attacks claimed by the Islamic State jihadist group.

A bar employee working near the cathedral raised the alert on Sunday after noticing a gas cylinder on the back seat of the car, which had no number plates, a police source said.

That cylinder was found to be empty but five full cylinders were found in the boot of the car.

No detonators were found, police said.

The car was parked in a side street opposite the cathedral on the left bank of the Seine.

The car’s owner was identified on Tuesday.

Notre Dame, a Gothic cathedral famous for its flying buttresses, stained glass windows and gargoyles, is one of Paris’s most popular landmarks, attracting 13 million visitors each year.

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.

Such attacks would create large numbers of victims without sacrificing suicide bombers, he told a parliamentary committee.

In July, 86 people were killed and dozens injured when a truck ploughed into a Bastille Day crowd in the Mediterranean resort of Nice. IS said the driver, Mohamed Lahouaiej Bouhlel, was one of its followers.

Less than two weeks later, two young jihadists murdered a priest near the northern city of Rouen.

French security services are particularly worried about the danger posed by extremists returning from Syria, where they have been fighting with IS forces.

France’s top prosecutor said last week around 700 French nationals were currently in Syria.