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 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 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 antiterrorism section of the Paris prosecutor’s office and that an attack had been “imminent.” Ms. Thibault-Lecuivre said, however, that the authorities “do not know where nor how” it was to be carried 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 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 the Islamic State in attacks in Paris and Brussels.
Ms. Agnès 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 on Tuesday was a small fraction of the amount found in a Brussels apartment shortly after 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 eight gallons of hydrogen peroxide. TATP is so unstable that even a small amount can cause a significant explosion.