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 taken into custody 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 that terrorists pose 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 the BFM TV news channel, although he did not comment specifically on the arrests.
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” the detainees planned to carry it out.
Three of the people taken into custody 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 in the search of the 20-year-old’s home, the police found 70 grams of TATP, or triacetone triperoxide, a peroxide-based explosive that was used by the Islamic State in past attacks in Paris and Brussels.
Thibault-Lecuivre said that police 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 thwarted plot, the amount of chemicals found Tuesday was a 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 highly unstable that even a small amount can cause a significant explosion.
France is under a state of emergency that was declared after the November 2015 attacks in and around Paris that left 130 people dead, and the country has been consistently on edge. Most recently, a man wielding two large knives was shot in Paris after he attacked a military patrol near the Louvre Museum.
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.
Le Roux said 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 the Cathedral of Notre-Dame in Paris with a car stuffed with gas canisters.
Two months later, the French authorities said that five people who had been taken into custody were Islamic State operatives being directed by a commander in Iraq or Syria, and that they were planning an “imminent” attack.Alissa J. Rubin, Benoît Morenne and Elian Peltier contributed reporting.