France opened a formal investigation Tuesday against three adults and a teenager accused of planning an attack on security forces in the coming days, which the French interior minister had said would have been “extremely violent”.

The four were arrested on Friday as part of an investigation into a terrorist conspiracy.

They were handed over to a court in Paris on Tuesday, where a judge opened a formal investigation against them, prosecutors said.

A day earlier, Interior Minister Christophe Castaner had said that the four suspects were being held “over a plan to carry out an extremely violent terror attack”.

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 offences, 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.

The source put the suspects’ ages at 17, 26, 38 and 39.

Investigators said they had become convinced that the attack may have been planned to coincide with the start of Ramadan, the Muslim month of fasting which starts next Sunday and Monday.

That was the reason why the authorities had decided to arrest them now, the source close to the inquiry said, adding that “possible scoutings had been made and (they) were looking to procure weapons”.

The Europe 1 radio station reported that a Kalashnikov rifle had been found among the weapons seized.

During the four days they had been held so far, the suspects had not contested their adhesion to radical Islamist thinking, the source said.

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 ISIS urging followers to target soldiers and police in France.

Despite the collapse of ISIS’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 were indicted in Paris on suspicion of planning an attack on a school or a police officer.

Last Update: Wednesday, 20 May 2020 KSA 09:52 - GMT 06:52