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French police arrested three "radicalized, fanaticized" women Thursday after a violent standoff in connection with a car parked next to Paris' Notre Dame Cathedral with six gas canisters inside, authorities said.

The women, ages 39, 33 and 19, were arrested in Boissy Saint Antoine in the Essonne region, southwest of Paris, French Interior Minister Bernard Cazeneuve said at a news conference. He said they appeared to be preparing "new violent and imminent actions."

An officer was being treated for neck and shoulder injuries after one of the women attacked him with a knife, Cazeneuve said, but the officer wasn't believed to be in danger. One of the suspects was also injured, he said, but no details of her injuries were made public.

The arrests bring to seven the number of people known to have been detained so far in a terrorist investigation that began with the discovery of the car on Sunday.

A car carrying gas canisters was discovered Sunday near Notre Dame Cathedral in Paris. AFP - Getty Images

The Paris prosecutor's office said earlier Thursday that police had arrested a couple — a 27-year-old man and a 26-year-old woman — on Wednesday in the Loiret region of France, south of Paris. A second couple — a 34-year-old man and a 29-year-old woman — were detained in the same case Tuesday.

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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 terrorism suspects for up to four days without charges.

"France is facing an unprecedented threat" from people recruited and indoctrinated by terrorists in Syria and Iraq, said Cazeneuve, who described Thursday's operation in Boissy Saint Antoine as "a race against the clock."

Prosecutors opened a terrorism investigation after the car was found near the famous cathedral 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, officials said.

No detonators were found in the car, which had been left on a narrow cobbled street in the city's 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.

France remains on maximum alert after calls for attacks on the country, which is taking part in bombing ISIS bases in Iraq and Syria.

Thousands of extra police and soldiers have been deployed to patrol sensitive sites since 130 people were killed by Islamist gunmen and suicide bombers in attacks on Paris last November.

A state of emergency declared at that time is still in place and gives police extra search and arrest powers, but debate still rages over security after another attack in July in which a man drove a truck into crowds in the city of Nice, killing 86.