Belgian police have made 16 arrests after a series of 21 raids nationwide, targeting “religious jihadism in a part of the Chechen community,” as the prosecutor’s office was quoted by AFP.

Belgian authorities initially said the 16 individuals would be detained for questioning. Bloomberg News reports that at least four of them have now been released, while two have been formally arrested, and three more arrest warrants have been issued.

The Wall Street Journal says some of these Chechens may have undergone military training and fought with Islamist groups, particularly the al-Qaeda-affiliated Nusra Front, and were “potentially planning attacks in Belgium.”

“The arrests are a sign of rising concern among European security services about radicalization in Europe’s Chechen diaspora,” the WSJ report continues. “France this year arrested more than a dozen Chechens suspected of terrorism, some for being part of a group that sent people to fight in Syria. Austrian police arrested nine people from the country’s large Chechen community last year who authorities said were on their way to Syria to fight with Islamic State.”

Reuters adds that in January, Belgian police “killed two men who opened fire on them during a series of raids against an Islamist group that federal prosecutors said was on the point of launching attacks across the country.”

According to the prosecutor’s office, U.S. intelligence assisted the Belgians by “monitoring messages exchanged by the suspects over WhatsApp, the widely used Internet messaging service.”

“The Belgian investigation began after authorities became aware of a man from Ostend who had returned to Belgium to seek medical care after being injured in Syria,” the Wall Street Journal reports. “The authorities uncovered a group of mostly Chechens who had experience fighting near Aleppo and were encouraging others to fight in Syria, the statement said.”

As Reuters describes it, the investigation gained urgency when two different groups of Chechens began exchanging ominous Internet communiques – one located in the university town of Leuven, the other located along the coast. The Leuven cell was apparently contemplating some sort of terrorist attack, although statements from the prosecutor’s office today do not portray this attack as “imminent.”