PARIS – Four people, including a 16-year-old girl, who were believed to be preparing a terrorist attack were arrested in southern France on Friday after bomb-making materials were found in the home of one of the detainees, according to the Paris prosecutor’s office.

The three other people arrested near the city of Montpellier were men ages 20, 26 and 33, said Agnès Thibault-Lecuivre, a spokeswoman for the prosecutor’s office, but no other information about the four was released.

The arrests highlighted the danger posed to France, which has been the site of several deadly attacks in the past two years and is thought to be the most targeted country in Europe. “We are facing an extremely high level of threat,” Prime Minister Bernard Cazeneuve said in an interview with BFM TV news.

In a statement, the Interior Ministry said that the police operation was preceded by a two-week investigation led by the anti-terrorism ­section of the Paris prosecutor’s office and that an attack had been “imminent.” Thibault-Lecuivre said, however, that the authorities “do not know where nor how” it was to be carried out.

Three of the people arrested were “directly suspected of preparing a violent act on our territory,” the interior minister, Bruno Le Roux, said in the statement.

The prosecutor’s office said that the police who searched the 20-year-old’s home found 70 grams of TATP, or triacetone triperoxide, a peroxide-based explosive that was used by ISIL in attacks in Paris and Brussels.

Thibault-Lecuivre said that police officers had also discovered ingredients that could have been used to produce more TATP, including a liter, or just over a quart, of acetone, a liter of hydrogen peroxide and a liter of sulfuric acid.

Although officials noted the serious nature of the threat posed by the thwarted plot, the amount of chemicals found Tuesday was a small fraction of the amount found in a Brussels apartment shortly after the deadly attacks in the Belgian capital last March.

Investigators in Brussels found 30 pounds of TATP in one apartment and ingredients to make much more, along with 40 gallons of acetone and 8 gallons of hydrogen peroxide. TATP is so unstable that even a small amount can cause a significant explosion.

State of emergency

France is under a state of emergency declared after the November 2015 attacks in and around Paris that left 130 people dead, and the country has been consistently on edge. This month, a man wielding two large knives was shot in Paris after he attacked a military patrol near the Louvre.

The prosecutor’s office said Friday evening that the man, who remained hospitalized, had been placed under formal investigation on terrorism charges, including attempted murder and criminal conspiracy.

Parliament sought to further bolster security last year by passing a series of measures that gave the judicial and police authorities more powers in dealing with terrorism suspects, but the government suffered a setback on Friday when France’s top constitutional court struck down one of the new provisions.

The court, the Constitutional Council, which ensures that legislation conforms to the French Constitution, found that a provision that made it a crime to regularly consult websites that promote terrorism disproportionately violated the freedom of ­communication.

The court ruled that existing legislation was sufficient to counter terrorist propaganda online. The provision had subjected offenders to a prison sentence as long as two years and a fine of up to $33,500.

French authorities regularly uncover plots to carry out attacks. Le Roux told lawmakers in December that 13 attempts to commit terrorist attacks had been thwarted over a five-month period.

The plots had involved more than 30 people, including women and minors, stressing that this amounted to a “very striking evolution in the course of the recent months” and that “the whole national territory is being ­targeted.”

The plots included a thwarted effort in September, when security forces disrupted a group of radicalized young women who planned to set off an explosion near Notre Dame in Paris using a car stuffed with gas canisters.

Two months later, French authorities said that five people who had been arrested were ISIL operatives being directed by a commander in Iraq or Syria, and that they were planning an “imminent” attack.

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