Belgium on Monday charged three people with involvement in a terrorist group after they were arrested in an operation at the weekend, the federal prosecutor said.

“They were charged with participation in the activities of a terrorist group,” the prosecutor’s office said in a statement, adding that a fourth person detained on Sunday had been released.

The three men were identified as Yassine A., Mohamed B. et Aboubaker O., the prosecutor said, adding that “in the interest of the inquiry,” further details would not be provided.

Belgian police had detained four people during a series of 13 raids on Sunday across the capital Brussels and the towns of Mechelen, about 30 kilometres (20 miles) due north of Brussels, and one in Duffel, 40 kilometres in the same direction.