PARIS—Police detained four people in southern France on Friday on suspicion of planning an attack using the same explosive employed by suicide bombers in Islamic State assaults in Paris and Brussels.

Three men—aged 20, 26 and 33—and a 16-year-old girl were taken into police custody in and around the city of Montpellier, according to people familiar with the investigation. At the home of one of the men, police found a liter each of acetone, hydrogen peroxide and sulfuric acid, chemicals used to make the explosive material TATP.

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