Four people, including a 16-year-old girl, were arrested on Friday by anti-terrorist police in the southern French city of Montpellier on suspicion of planning a terrorist attack, police and judicial sources said.

“An attack has been foiled,” said the sources, adding that the four were arrested after buying acetone, a highly flammable liquid that can be used to make bombs.

“It seems that they intended to go through with it and to make several explosive devices,” the sources said.

Those in custody included a 22-year-old man and his 16-year-old girlfriend, both known to authorities for connections with extremism, after police found TATP explosives and other bomb-making materials in his home.

A police official said one of the suspects was believed to be planning a suicide attack but that the investigation had not yet uncovered a specific target.

France remains on high alert after a wave of attacks which began two years ago that has claimed more than 200 lives. The country has been under state of emergency rules since November 2015.

Last week, a soldier shot and wounded a machete-wielding attacker who lunged at him outside the Louvre museum in Paris.

Investigators have identified the man as an Egyptian and are trying to establish whether he had any link to a jihadist group.