Belgian and French officials want to know where Salah Abdeslam is, and Belgium has issued an international warrant for him.
Jean-Pascal Thoreau, a spokesman for Belgium's Federal Prosecutor, said Abdeslam is one of three brothers suspected of involvement in the attacks. One of the brothers was killed in the attacks, and another was arrested by Belgian police, Thoreau said.
Salah Abdeslam had been questioned by French police earlier but was not detained, a source close to the investigation into the Paris attacks said.
He was driving in the direction of the Belgian border when stopped by police and questioned a few hours after the attacks, the source said. Now, his whereabouts are unknown.
The Paris attacks killed at least 129 people
and wounded more than 350 others, many of them seriously.
The Islamic terror group ISIS has claimed responsibility for the slaughter, which France's President described as "an act of war."
French fighter jets struck ISIS targets in the Syrian city of Raqqa on Sunday
in what officials called a major bombardment and analysts described as a retaliatory move. Authorities made a number of arrests linked to the attacks in Belgium, and investigators homed in on an abandoned car with weapons found in eastern Paris.
Two days after the Paris attacks, the French capital is a city in mourning -- and on edge -- with key questions looming: Do suspects remain at large? Could another attack be in the offing? And how will France and other countries respond?
Hiding among refugees?
It's possible suspects who were directly involved in the attacks remain at large, a French counterterrorism source close to the investigation told CNN.
While investigators pieced together information about who could be on the loose, details emerged about several of the seven attackers who authorities say were killed:
• At least three of the terrorists involved in the attacks had spent time in Syria, a French official told CNN Sunday. The official did not identify the attackers.
• One of three bombers who detonated themselves at the Stade de France late Friday arrived on the Greek island of Leros on October 3 among numerous Syrian refugees, CNN's Christiane Amanpour reported
, citing an unnamed French senator who was briefed by the Ministry of the Interior.
The senator told CNN that the man was carrying a fake or doctored Syrian passport, and also a registration document for refugee status issued by Greek authorities. The fingerprints taken by Greek authorities match those of the terrorist who blew himself up at the Stade de France, the senator said.
• Bilal Hadfi, a Belgian resident who is thought to have fought in Syria, was one of the attackers, according to several sources. Hadfi, who was killed in the attacks, was 19 or 20, the sources said.
• One of the suicide bombers has been identified as Ismael Omar Mostefai
, according to Jean-Pierre Gorges, the mayor of the French town of Chartres, who is also a member of Parliament. Mostefai lived in Chartres at least until 2012, Gorges said in a Facebook post Saturday
Paris prosecutor Francois Molins described him as a 29-year-old French citizen with a criminal record from the southern Paris suburbs. The attacker, who was involved in the assault and hostage-taking at the Bataclan concert hall
, was identified by fingerprints and was believed to have been radicalized in 2010 but had never been accused of terrorism, Molins said.
Agnes Thibault-Lecuivre, the Paris prosecutor's spokeswoman, told CNN that six people -- all family relatives of Mostefai -- have been detained. It is common practice in criminal cases in France to place family members in custody. Mostefai's relatives have not been charged or arrested.
City on edge
The violence that hit six sites around the city has deepened the trauma for Parisians, who had already been shaken by a series of terrorist attacks in January
that left 17 people dead.
People were still out around the city Saturday, some posing for selfies along the Seine or stealing a kiss in front of the Eiffel Tower. But the tower itself -- like many other tourist attractions, shops and public buildings -- has been closed
in the aftermath of the attacks.
Reflecting just how nervous Paris is right now, a panic broke out Sunday evening in a crowd gathered at a memorial erected at one of the sites of Friday's terror attacks.
Video showed mourners suddenly spooked, though it's still unclear why.
People ran away screaming. Some jumped over lit candles of the memorial, others grabbed their children and sprinted away.
The panic ended quickly and police cleared the memorial, saying no one is allowed to gather, at least for the time being.
Security has been stepped up, with military reinforcements drafted in.
There has been an outpouring of support and solidarity in Paris, but large gatherings in the streets have not materialized, partly because the state of emergency prohibits them.
Sadness and fear are taking a toll, Paris Deputy Mayor Patrick Klugman said.
"We don't know yet if this terror in fact is over or not," he told CNN, highlighting the jitters caused by unsubstantiated rumors circulating on social media.
Three Kalashnikovs found in abandoned car
CNN affiliate BFMTV
reported Sunday that authorities had found three Kalashnikov automatic rifles in an abandoned car in the eastern Paris suburb of Montreuil that was believed to have been used by perpetrators of the attacks.
It's unclear whether any arrests authorities have made so far are connected to the vehicle, the French counterterrorism source said.
At least seven people have been arrested in raids in Belgium connected to the Paris attacks, officials said. Those arrested were in contact with the Paris attackers, a senior Belgian counterterrorism source told CNN. No weapons or explosives were found on them.
Raids were conducted in a Brussels suburb, Justice Ministry spokeswoman Sieghild Lacoere said. A car rented in Brussels was found near one of the sites of the Paris attacks, which triggered the raids, Lacoere said.
On Saturday, CNN affiliate BFMTV reported that the father and a brother of one of the attackers had been taken into custody. And AFP reported that the two men were detained after police raided their homes 130 kilometers (80 miles) east of Paris. CNN has not independently verified that the men were picked up by authorities.