A 15-year-old boy was arrested in Paris on Wednesday after authorities discovered his plans to carry out an act of terrorism, making him the third minor in less than a week to be apprehended.

Judicial sources said the boy appears to be part of the same network of radicalized individuals as two other adolescents arrested and jailed over the weekend, one in Paris and one just west of the city.

He was placed under watch and questioned about possibly belonging to a “group of delinquents with ties to a terrorist group,” French TV station BFMTV said.

The teen was arrested in Paris’ 20th arrondissement, or district, a neighborhood known to be one of the city’s least affluent, and home to large immigrant communities.

Authorities searched his home on Wednesday. They were allegedly aware of the teenager’s radicalized activity, according to BFMTV. He was filed under the “S” category, meaning that he was being monitored by the state.

It’s work that “is keeping us busy night and day,” interior minister Bernard Cazeneuve said on Wednesday. “Our investigation work is more intense than it’s ever been given the heightened threat level and the calls, coming from various players based in Syria who communicate via encrypted messaging, for French people to carry out attacks.”

There are about 20,000 people who fall under the “S” category in France, roughly 10,000 of whom have shown ties to radical Islam.

"Our investigation work is more intense than it's ever been given the heightened threat level and the calls, coming from various players based in Syria who communicate via encrypted messaging, for French people to carry out attacks." Bernard Cazeneuve, French interior minister

The cases of all three minors are connected. They all used Telegram, an encrypted messaging service, to receive guidance on attack planning. They’d also all been in contact with Rachid Kassim, a French jihadi well known to authorities. Kassim is suspected of having mentored the boys via social media from somewhere inside a part of Syria or Iraq controlled by the so-called Islamic State, HuffPost France reported.

Kassim was allegedly linked to the car full of explosives placed in central Paris by female commandos on Friday. He is also believed to have been involved in the murder of two French police officers in June and a priest in July.

“Women and sisters are executing attacks. So where are the brothers? … If these women are ready to act, it’s because there are too few men who are ready to act,” Kassim said in a Telegram message quoted by Le Monde.

Ralph Orlowski / Reuters German special police forces escort a Syrian suspected of being members of Islamic State outside the building of the German Federal Supreme Court (Bundesgerichtshof) in Karlsruhe, Germany, Sept. 13, 2016.

Authorities across Western Europe are being kept busy staving off attacks, a mission of unprecedented importance given the recent terror attacks in Paris, Nice and Brussels. German police arrested three Syrian young men on Tuesday suspected of being instructed by ISIS to carry out an attack.

The trio arrived in late 2015, likely thanks to the same smuggler network that funneled militants into Paris to carry out November’s attacks, according to interior minister Thomas de Maiziere.