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 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.