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.
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.
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.
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.
Abdeslam and his older brother, Ibrahim, who blew himself up at a Paris cafe during the deadly rampage, both hailed from Molenbeek. So did Abdelhamid Abaaoud, the suspected ringleader of the attacks, who authorities say was killed in a police raid near Paris on Wednesday.
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.
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.
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.
French officials, meanwhile, have reported a surge in people applying to join the military following the Paris attacks and Hollande's declaration that the nation is "at war" against ISIS.