Four people suspected of being "radical Islamists" have been detained in a terrorist investigation stemming from the discovery of a car parked next to Paris' Notre Dame Cathedral with six gas canisters inside.

The Paris prosecutor's office said Thursday that police arrested a couple -- a 27-year-old man and a 26-year-old woman -- a day earlier in the Loiret region of France, south of Paris.

A second couple -- a 34-year-old man and a 29-year-old woman -- was detained in the same case on Tuesday.

The two men, who are brothers, and the two women were transferred to Paris to be questioned by investigators trained in counterterrorism. The prosecutor's office said all four are suspected of links to "radical Islamism".

Authorities are allowed to hold terror suspects for up to four days without charges.

Prosecutors opened a terrorist investigation after the car was found near the famous cathedral on Sunday morning. No one was inside, but police found five canisters filled with gas in the trunk and an empty canister on one of the seats. Along with the canisters, there were three jerry cans of diesel and papers with Arabic writing inside the vehicle.

No detonators were found in the car, which had been left on a narrow cobbled street in the popular Latin Quarter next to bars and restaurants.

The car's owner went to police to report that his 19-year-old radicalized daughter was missing. He was briefly detained and then released.

The two arrested couples have been living in the Loiret region, in the area of Montargis town. In March, three members of a family were arrested in the same area on suspicion of being members of a terrorist network.

More people are still being sought in the Notre Dame case, the prosecutor's office said.