PARIS, France — Three adults and a teenager have been arrested over a planned attack on the security forces in the coming days, prosecutors said Monday, which a minister described as “extremely violent.”

All four were arrested on Friday as part of an investigation into a terrorist conspiracy, but there were no immediate details on the nature of the planned attack.

“Four people are being held over a plan to carry out an extremely violent terror attack,” Interior Minister Christophe Castaner told reporters.

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One of the suspects is a minor serving a probationary sentence at an educational facility after being sentenced to three years, two suspended, for trying to travel to Syria, the Paris prosecutors office said.

The other three are known for common law offenses, according to a source close to the inquiry, which was opened by prosecutors on February 1 and is being handled by the DGSI, France’s domestic intelligence agency.

France has been on high alert since the start of a wave of jihadist attacks began in 2015, leaving more than 250 people dead, with the Islamic State (IS) group urging followers to target soldiers and police in France.

Despite the collapse of IS’s self-proclaimed “caliphate” in Iraq and Syria last month, the threat of further attacks inspired by the group remains high.

At the end of March, two men, one with psychiatric problems, were indicted in Paris on suspicion of planning an attack on a school or a police officer.