Belgian authorities charged an additional suspect Monday in connection with this month’s deadly Paris attacks, as Belgium entered the workweek with shuttered schools and offices in an effort to disrupt a suspected similar plot.

The suspect, whose name was not released, was one of 21 people detained in 29 raids in the capital and the southern cities of Liege and Charleroi, a sweep that ended Monday morning. Other arrests appeared less fruitful; 17 people were released. The suspect was charged with “participating in activities of a terrorist group and a terrorist attack (Paris),” the Federal Prosecutor’s Office said in a statement.

While three others were still being questioned Monday night, prosecutors said they had not located the main suspect sought in the Nov. 13 attacks in the French capital: Salah Abdeslam, 26, a French national born in Belgium.

Belgian officials think Abdeslam, one of at least nine men suspected of involvement in the deadliest rampage on French soil since World War II, is at large in Belgium.

Belgian Prime Minister Charles Michel, seeking to avert what officials said was a plot to launch similar coordinated attacks in Brussels, said the threat remains “serious and imminent.” Speaking at a news conference Monday night, he said Belgium will remain on its highest alert level. Soldiers who have deployed at hotels and subway stations will stay on the streets for at least a week, he said.

Authorities plan to reopen schools Wednesday. The Brussels metro system will begin gradually reopening then, although “serious attention” will be paid to security on the network, Michel said. Extra security personnel will be deployed at schools.

In the wake of the assaults near a stadium, inside a concert hall and at other sites across Paris, European nations have imposed new security measures, and France has called on its allies to mobilize military and diplomatic might against the Islamic State. The extremist group claimed responsibility for the onslaught, which killed at least 130 people and wounded more than 350. Seven attackers also died.

Europe’s heightened anxiety has been visible in the past few days in Belgium, where the streets were eerily quiet Monday. Most companies advised their employees to work from home.

“We will continue as long as necessary,” Interior Minister Jan Jambon said earlier in the day. “We have to make sure that we roll this entire network.”

Meanwhile, the State Department issued a worldwide travel alert Monday, warning U.S. citizens that the Islamic State, al-Qaeda and other groups “continue to plan terrorist attacks in multiple regions.”

Remembering the victims of Friday’s attacks in Paris

The known Paris attackers have been identified as young French and Belgian men whose support for the extremist cause took shape in Europe. Some fought in the ranks of the Islamic State in Syria, including Abdelhamid Abaaoud, a Belgian who is thought to have headed attack planning in Europe. He died in a police raid Nov. 18 in a northern Paris suburb.

After the attacks, European leaders are examining security and border controls at home. Several of the attackers are thought to have entered Europe on a migrant trail through Greece. In France, President François Hollande has won support for a three-month extension of a state of emergency, and he now plans to pursue changes to France’s constitution.

French investigators are trying to piece together a clear picture of how the attacks came together. Monday evening, the Paris prosecutor’s office said an explosive vest was found discarded in Montrouge, a southern suburb.

Hollande is also vowing to intensify France’s military response to the Islamic State. Monday, the French military said its planes began conducting airstrikes against the group’s positions in Iraq from the eastern Mediterranean, where the aircraft carrier Charles de Gaulle arrived over the weekend.

Earlier Monday, Hollande hosted British Prime Minister David Cameron, who vowed to seek parliamentary support for expanding British airstrikes into Syria.

Hollande plans further meetings this week with President Obama and leaders from Germany and Russia on strategies against the Islamic State alongside efforts to end Syria’s nearly five-year civil war.

Ryan reported from Paris. Annabell Van den Berghe in Brussels; Cléophée Demoustier, Virgile Demoustier and Griff Witte in Paris; and William Branigin and Brian Murphy in Washington contributed to this report.

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