PARIS -- An international manhunt is underway for a French citizen who is suspected of involvement in the deadly Paris terrorist attacks and has already managed to slip through the fingers of police at least once.
The French government is stepping up its response to Friday night's bloodthirsty rampage across Paris by attackers armed with Kalashnikov assault rifles and suicide vests who massacred at least 129 people.
French warplanes on Sunday bombarded Raqqa, the Syrian city that serves as the de facto capital for ISIS, the Islamic extremist group that has claimed responsibility for the attacks. In France, where a state of emergency is in effect, police carried out raids around the country overnight into Monday, seizing weapons that included a rocket launcher, officials said.
Details have started to emerge about the people believed to have carried out the coordinated Paris attacks, which have heightened fears about ISIS' ability to strike at the heart of Western nations. Some of the suspects have been linked to the bloody civil war raging in Syria that has enabled ISIS to thrive.
"The attack was organized, conceived, and planned from Syria," French Prime Minister Manuel Valls said Monday in a radio interview.
He warned that France needed "to prepare ourselves for further attacks."
"Terrorism can strike, has struck -- though we have avoided several attacks -- and could strike again, now, in the coming days, in the coming weeks."
Suspect at large described as dangerous
Seven of the attackers who carried out the Paris attacks are dead, and at least one man suspected of involvement is at large, according to authorities.
The suspect, Salah Abdeslam, is a 26-year-old French citizen who was born in Belgium, French police said in a public request for information, warning that he is dangerous and not to be approached. Belgium has issued an international warrant for his arrest.
Investigators haven't said much about how they believe Abdeslam is tied to the shootings and bombings that targeted people at restaurants, bars, a concert hall and a sports stadium.
The French newspaper Le Monde reported that he rented the black Volkswagen Polo that was found outside the Bataclan concert hall where three attackers massacred at least 89 people before blowing themselves up or being shot by police.
Questioned by police, then let go
Jean-Pascal Thoreau, a spokesman for Belgium's federal prosecutor, said Abdeslam is one of three brothers suspected of involvement in the rampage of violence. One of the brothers was killed in the attacks, and another was arrested by Belgian police, he 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 a few hours after the attacks when officers stopped him, the source said. Now, his whereabouts are unknown.
Le Monde reported that police hadn't yet linked him to the Paris attacks when they stopped him and two other people in a black Volkswagen Golf. When Belgian police stopped the car later Saturday, Abdeslam was no longer in it.
On Monday, police had blocked off two streets in an active standoff in Molenbeek, a suburb of Brussels, Belgium, with a history of links to terrorism plots. Police in balaclavas surrounded a building and were using a megaphone to command someone to come out, a CNN team on the ground reported.
In the aftermath of the raid, the Belgian Federal Justice Department said that police had made one arrest, but had not apprehended Abdeslam, Belgian state broadcaster RTBF reported.
Car found with weapons inside
Le Monde reported that his older brother, Ibrahim Abdeslam, was the suicide bomber whose explosives detonated at a cafe on boulevard Voltaire in eastern Paris during the wave of attacks on the city. The Paris prosecutor's office has identified that attacker as a 31-year-old French citizen but hasn't disclosed his name.
According to Le Monde, Ibrahim Abdeslam rented the black Seat car that authorities say was used in the string of deadly attacks on restaurants and bars on Friday. But it's not yet clear whether he was in the vehicle at the time of the attacks, the newspaper said.
The Seat was found abandoned in the eastern Paris suburb of Montreuil with three Kalashnikov automatic rifles inside, CNN affiliate BFMTV reported Sunday.
Other suspected attackers have been identified, including Ismael Omar Mostefai, a 29-year-old French citizen from the Paris suburbs who authorities say was radicalized in 2010 but wasn't known to be associated with a terrorist group. Mostefai was one of the three terrorists who stormed the Bataclan concert hall, according to officials.
On Monday, the Paris prosecutor's office identified another of the Bataclan attackers as Samy Amimour, a 28-year-old from the Parisian suburb of Drancy.
Amimour was known to have links to terrorists and had been the subject of an international arrest warrant since 2013 after violating the judicial supervision he had been placed under, the prosecutor's office said in a statement.
He was placed under supervision in 2012 after anti-terrorism authorities investigated an aborted attempt to travel to Yemen, the statement said.
Stadium attacker believed to have fought in Syria
Authorities have also released names for two of the suicide bombers who set off their explosives outside the Stade de France, the national stadium where France's soccer team was playing Germany.
One is Bilal Hadfi, who is reported to have been a 20-year-old French citizen living in Belgium.
Guy van Vlierden, a Belgian terrorism expert, said Hadfi is thought to have fought in Syria, where he went by the names Abu Moudjahid Al-Belgiki and Bilal Al Mouhajir.
Hadfi appears to have traveled to Syria in the spring of 2015, van Vlierden said, citing analysis of his social media postings and other communications.
At least three of the terrorists involved in Friday's attacks in France have spent time in Syria, a French official told CNN Sunday. The official did not specify who those three attackers were.
Syrian passport holder linked to refugee flow
Attention is also heavily focused on a Syrian passport found near the body of another of the three Stade de France bombers.
The first stadium attacker was carrying the passport, a French senator who was briefed by the Interior Ministry told CNN's Christiane Amanpour. French officials have determined that the bomber was among a group of Syrian refugees who arrived on the Greek island of Leros on October 3.
The senator told CNN that the man was carrying the passport and also a registration document for refugee status by Greek authorities. The fingerprints of the passport holder taken by Greek authorities match those of the terrorist who blew himself up at the Stade de France, the senator said.
The Paris prosecutor's office on Monday confirmed the link, saying the passport bearing the name of Ahmad Al Mohammad, a 25-year-old from Idlib, still needs to be verified.
Refugee concerns intensify
The alleged link between one of the attackers and the wave of refugees from the Syrian war flocking to Europe this year has intensified concerns about how to handle the massive influx of people.
"The fear that terrorists are hiding amongst refugees will increase and will be used by anti-immigrant politicians," said Karen Jacobsen, who directs the Refugees and Forced Migration program at Tufts University's Feinstein International Center.
But she said measures like "sealing borders with razor wire and having police on trains requesting papers" won't prevent desperate refugees from reaching Europe.
"The United States and Europe must work together to find reasonable ways to manage the current refugee flow into Europe and to utilize more creative and effective intelligence to monitor terrorist plans," Jacobsen wrote in an opinion article for CNN.
Raids across France
Police carried out a series of anti-terrorism raids early Monday in France, including in suburbs of Paris, Toulouse and Grenoble, French media reported.
Valls, the French Prime Minister, said the raids in various locations targeted people on a special watch list, adding that guns, money and marijuana were found and seized.
Twenty-three people are in custody, French Interior Minister Bernard Cazeneuve said Monday, adding that a rocket launcher and bulletproof vest are among the items seized. He said he had ordered that 104 people be put under house arrest since the attacks.
Valls said more than 150 police raids have been carried out under the state of emergency across France since Friday.
At least seven people were arrested in weekend 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.