A criminal terrorist investigation has been opened in Paris following the discovery of a car parked near Notre Dame Cathedral with seven gas canisters and pages written in Arabic inside, prosecutors said.
The Paris prosecutor's office revealed on Wednesday that a couple it described as radicalised - a 34-year-old man and a 29-year-old woman - were arrested a day earlier in a highway rest area near the southern town of Orange and transferred to Paris to be questioned in the case.
The car found near the famous cathedral on Sunday morning had its licence plates removed and hazard lights on.
That evening, its owner went to the police to report that his radicalised daughter was missing but without saying his car had also disappeared, the prosecutor's office said.
Police briefly detained and questioned the car owner before letting him go, the prosecutor's office said. His daughter is still being sought, the office said.
The man and woman arrested on Tuesday were already known by French security services for their alleged links with "radical Islamism", prosecutors said. The prosecutor's office would not say why investigators suspect them in the case nor whether the couple might have been in contact with the car owner's daughter.
The papers written in Arabic are being analysed, the prosecutor's office said.
It said it has opened an investigation under suspicion of a "criminal terrorist association".
Two others officials said an employee of a bar near Notre Dame Cathedral reported the car on Sunday morning as it was parked along the Seine River.
No-one was inside, but police found six canisters filled with gas in the boot and an empty canister on one of the seats.
No detonator or ignition materials were found in the car, the prosecutor's office said.
In addition to the car owner, three other people were briefly detained and questioned in the case before being released, it said.
French interior minister Bernard Cazeneuve said it was still not clear why the car was abandoned or what the alleged intentions were of those under arrest.
Mr Cazeneuve said there have been 260 arrests linked to extremist networks since the beginning of the year "and a significant number of these people were preparing attacks".
In an interview with Le Monde newspaper earlier this week, Paris prosecutor Francois Molins said an increasing number of teenage girls have been radicalised, with "very worrying profiles", ''very harsh personalities" and "sometimes terrorist plans that, intellectually, start to be brought to completion".
Mr Molins said 35 minors - 23 boys and 12 girls - are being investigated under preliminary charges, nine of whom have been detained.
France is on alert after a deadly string of Islamic State attacks and threats against landmarks.
Algeria-linked extremists used gas canisters filled with nails during attacks on Paris in the 1990s.