Before the raids Friday night, the most recent terror-related detention in Belgium had come earlier in the week, when federal prosecutors announced the detention and investigation of a man identified as Youssef E. A., 30, in connection with the March attacks in Brussels. He was charged on Friday with “participation in a terrorist group, terrorist murders and attempted terrorist murders as a perpetrator, co-perpetrator or accomplice.”

This month, another man, Ali E. H. A., 31, was similarly charged. The broadness of the charges in both cases suggests that the authorities have yet to determine what role the two men may have played.

After the Paris attacks, the Belgian authorities were accused of being insufficiently vigilant when it emerged that the attacks had been planned in Belgium and that the explosives had been manufactured there.

Most of the attackers were Belgian or French citizens.

Adding to the pressure on the Belgian authorities was information disseminated this month by the Belgian Coordinating Body for Threat Analysis. That government body, which evaluates intelligence and other terrorism-related information, sent an alert to the police saying extremists who had fought in Syria were headed for Belgium and France.

While the warning was based on “raw intelligence,” according to the Belgian authorities, its wide distribution to police services and its leak to a Belgian French-language newspaper suggested that it was being taken very seriously.

In the course of questioning detainees thought to be connected to the Paris and Brussels attacks, investigators in Belgium have picked up several references to the possibility of attacks during the Euro 2016 soccer tournament in France, which lasts through July 10. Both France and Belgium are on high alert.

Meanwhile, in the case of a police officer and his female companion who were stabbed to death in France on Monday night, the prosecutors’ office announced that it had opened an investigation and placed two people in preventive detention: Charaf-Din Aberouz and Saad Rajraji. Both men had been sentenced along with the presumed killer of the couple — Larossi Abballa, who was fatally shot — in a 2013 terrorism case but were released from prison.

They are facing preliminary charges of participation in a terrorist group that intended to commit one or many crimes.