Brussels, Belgium (CNN) Paris and Brussels continued to defend against the threat of terrorist attacks Sunday , with raids and arrests in the Belgian capital and a fresh appeal from French police.

It has been nine days since a wave of ISIS attacks in Paris killed 130 people . French authorities have said the Paris attacks were organized in Belgium.

According to Belgian federal prosecutor Frederic Van Leeuw, authorities carried out some 20 searches Sunday and 16 people were arrested. No firearms or explosives were found.

Also not found: At least one man suspected of involvement the Paris violence, Salah Abdeslam , a French citizen who grew up in Belgium, remains at large. The prosecutor did not identify any of those people in custody.

Meanwhile, the French National Police posted a picture of a man it said was one of the dead Paris attackers on its official Twitter account The photo is accompanied by an appeal to the public for information about the man, whom officials do not name.

"This individual is the third perpetrator who died in one of the attacks committed on November 13 at the Stade de France," the alert reads.

It asks the public to alert police if any information about the deceased suspect is known.

Children in France on Monday will have their bags searched at the front doors of schools, while administrators will be tasked with verifying the identities of everyone on school grounds, according to the Ministry of National Education website.

Gatherings around the front doors of schools are discouraged, the ministry said, explaining parents should drop off their kids and promptly leave and principals should designate on-campus sites where students can gather and older students can smoke. Field trips to the Paris area are canceled until next week, though trips within the Paris region can resume, the ministry said.

And while French officials scrambled to keep citizens safe, some took safety into their own hands, to a degree: French Samaritan Croix Blanche, an association of aid workers, announced that a record number of people were signing up for lifesaving classes.

Brussels subways remain closed

Belgium's capital will remain at the country's highest terrorism alert level, Belgian Prime Minister Charles Michel said Sunday evening. Authorities warned Friday night of a possible imminent threat to the capital which Michel said remains of the highest concern.

In Brussels, the subway remained closed, with only the city's above-ground trams and buses operating. People have been warned to avoid large gatherings.

Specific reasons for the extraordinary alert in Brussels weren't disclosed, but Michel said Saturday that authorities had reason to suspect possible attacks in more than one location.

"We are talking of a threat of several individuals with weapons and explosives, to launch acts, maybe even in several places at once," Michel said, evoking chilling similarities with the Paris attacks that hit restaurants, a rock concert and the area outside a sports stadium.

Michel said the authorities' main objective is to reduce the number of large events so police officers can be freed up to secure Brussels. A top-tier soccer game that was due to be played 50 miles outside the capital was canceled because of the need for police inside the city.

Manhunt for suspect

Authorities in Belgium have carried out a series of security raids in the past week related to the Paris attacks, looking in particular for Abdeslam, the 26-year-old suspect who's still on the run and described as dangerous.

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The increased threat level over the weekend, however, appeared to go well beyond the manhunt for Abdeslam, who was last seen by authorities in the hours after the Paris attacks.

French police stopped him and two other men in a car heading toward the Belgian border, but let them go because he hadn't yet been connected to the massacres in Paris.

Abdeslam and several other suspects have strong ties to Brussels, notably its suburb of Molenbeek, which has a history of links with terrorism plots

Photos: The search for Paris terror suspects A Belgian soldier stands guard around a security perimeter as a reported police intervention takes place around the Grand Place central square in Brussels on Sunday, November 22. Multiple raids took place throughout Brussels, according to the Belgium Federal prosecutor, and 16 people were arrested. Salah Abdeslam, a suspect in Paris terror attacks, was not among them. Hide Caption 1 of 16 Photos: The search for Paris terror suspects Belgian police officers and soldiers secure an area as a reported police intervention takes place around the Grand Place central square in Brussels on November 22. Hide Caption 2 of 16 Photos: The search for Paris terror suspects Belgian national Ahmad Dahmani, center, suspected to be involved in the November 13 Paris attacks, was arrested late Friday, November 20, in southern Antalya province, Turkey. Dahmani had been tracked by a police anti-terror unit after he arrived at the Antalya International Airport -- on a flight from Paris -- the day after the massacre. Hide Caption 3 of 16 Photos: The search for Paris terror suspects Two Syrian nationals, Ahmet Tahir, left, and Muhammed Verd, were arrested by Turkish security forces in southern Antalya province, Turkey, on November 20. They were captured, allegedly with a fake Syrian passport for Belgian national Ahmad Dahmani, suspected of location scouting for the Paris attackers. Hide Caption 4 of 16 Photos: The search for Paris terror suspects Police searched the home of Hasna Ait Boulahcen's mother in Aulnay-sous-Bois, France, on Thursday, November 19. Ait Boulahcen, 26, was killed during Wednesday's raid on an apartment in the Paris suburb of Saint-Denis, official sources in France told CNN. She did not blow herself up as was previously thought. Hide Caption 5 of 16 Photos: The search for Paris terror suspects A body is removed from an apartment that was raided by police in the Paris suburb of Saint-Denis, France, on Wednesday, November 18. French special forces were looking for those behind the November 13 terrorist attacks in Paris. The hourslong ordeal ended with at least two suspects dead and eight detained. Hide Caption 6 of 16 Photos: The search for Paris terror suspects Authorities zeroed in on the Saint-Denis building after picking up phone conversations that a relative of Abdelhamid Abaaoud, the attacks' purported ringleader, might be there. The Paris prosecutor's office later announced that Abaaoud's bullet-riddled body was found after the raid. Hide Caption 7 of 16 Photos: The search for Paris terror suspects French Interior Minister Bernard Cazeneuve visits Saint-Denis during the raid on November 18. Hide Caption 8 of 16 Photos: The search for Paris terror suspects A Renault Clio with Belgian license plates is towed by the police in Paris on Tuesday, November 17. The car is believed to have been rented by Salah Abdeslam. Authorities are looking for Abdeslam, a Belgium-born French national who is one of three brothers suspected in the terror attacks. Hide Caption 9 of 16 Photos: The search for Paris terror suspects Special forces stand guard on a roof in Molenbeek, a suburb of Brussels, Belgium, on Monday, November 16. Hide Caption 10 of 16 Photos: The search for Paris terror suspects Armed police guard a street in Brussels on November 16. Hide Caption 11 of 16 Photos: The search for Paris terror suspects Salah Abdeslam's brother Mohammed speaks to the media at his house in Molenbeek on November 16. Mohamed Abdeslam was released by police after being detained over the weekend. Hide Caption 12 of 16 Photos: The search for Paris terror suspects Special forces sit on a roof in Molenbeek as they prepare to enter a house on November 16. Hide Caption 13 of 16 Photos: The search for Paris terror suspects Armed police guard a street in Molenbeek on November 16. Hide Caption 14 of 16 Photos: The search for Paris terror suspects A man is detained by a police officer after a raid in the Mirail district of Toulouse, France, on November 16. French Prime Minister Manuel Valls said there were 150 police raids overnight in the country. Hide Caption 15 of 16 Photos: The search for Paris terror suspects Police officers overlook Paris as they patrol in front of the Sacre Coeur Basilica on November 16. Hide Caption 16 of 16

Belgian arrested in Turkey

Investigators are still trying to gather a full picture of who played what role in the shootings and bombings across Paris.

Adding to the complex picture, Turkish authorities have arrested three people with suspected ties to ISIS, including a Belgian man who they believe was in contact with the Paris attackers, a Turkish official said.

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Ahmet Dahmani, 26, a Belgian national of Moroccan descent, was arrested at a hotel in Antalya, CNN Turk reported. Two other suspects, Syrian citizens Ahmet Tahir, 29, and Mohammed Verd, 23, were arrested after they traveled from Syria to meet Dahmani, authorities said. The two were going to transport him to Syria, authorities said.

In France, investigators are analyzing the DNA of a third person who was killed during the major police raid Wednesday in the Paris suburb of Saint-Denis that targeted Abaaoud, the suspected ringleader.

Two of the dead -- Abaaoud and one of his female relatives, Hasna Ait Boulahcen -- have been identified. But the DNA of the third person, who is believed to have detonated a suicide device, doesn't match anyone on police records, according to CNN affiliate BFMTV.

France's TF1 network published a video clip of the raid on the Saint-Denis apartment where Abaaoud and Boulahcen died. The footage appears to have been filmed nearby and captures commandos and a woman's voice, presumably Boulahcen.

She screams, "I want to leave!"

A French commando orders, "Raise both of your hands! ... Shut your mouth!" and then another says, "Where is your boyfriend?"

The woman shouts, "He's not my boyfriend!"

"Where is he?" a commando shouts.

She again yells, "He's not my boyfriend!"

"She's not alone. ... She's not alone," a commando says.

The woman shouts, "Can I come out? Let me come out!"

Immediately after the raid, officials thought that Boulahcen had been wearing a suicide belt. But it later emerged that she was not. There were two others in the apartment with her -- Abaaoud and the other man, whom authorities have not yet identified.

Police are extending until at least Monday the detention of Jawad Bendaoud, who rented out the apartment in Saint-Denis where Abaaoud and the others were killed during last week's raid, said Agnes Thibault-Lecuivre, a spokeswoman for the Paris prosecutor's office.

Bendaoud is the only man still in custody related to the Saint-Denis raid. Seven of the eight people taken into custody Wednesday were released Saturday, Thibault-Lecuivre said.

French aircraft carrier joining campaign against ISIS

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The French government has responded to the Paris attacks by imposing a state of emergency for three months, giving security services a range of extra powers.

Police have conducted hundreds of raids on suspected Islamic militants, and scores of people have been detained or placed under house arrest.

The French military has also stepped up its bombardment of ISIS targets in Syria, launching waves of airstrikes on the militant group's self-declared capital, Raqqa.

France was already part of the U.S.-led coalition conducting an aerial campaign against ISIS in Syria and Iraq. But in the aftermath of the Paris massacres, President Francois Hollande announced that the aircraft carrier Charles de Gaulle would be deployed to the region, adding additional firepower.

The warship will be operational on Monday, French Defense Minister Jean-Yves Le Drian said in a radio interview Sunday.