BRUSSELS — The hunt for those responsible for the Paris terrorist attacks escalated on Monday as French officials identified a 27-year-old Belgian who fought for the Islamic State in Syria as the chief architect of the assaults and the police in France and Belgium conducted extensive raids seeking other suspects.
Three days after the attacks, which killed 129 people, French and Belgian security services were focused on the role of the Belgian, Abdelhamid Abaaoud, who is among the most prominent Islamic State fighters to have come out of Belgium and has been linked to a series of previous terrorist plots.
A French official briefed on the investigation, who spoke on the condition of anonymity because he was not authorized to discuss operational details, said Mr. Abaaoud had mentioned plans to attack “a concert hall” to a French citizen who returned from Syria.
Mr. Abaaoud, this official said, had also been in contact with Ismaël Omar Mostefaï, one of the Paris attackers. Mr. Abaaoud also knew another attacker, Ibrahim Abdeslam; they were tried together in 2010 in Belgium for a minor offense.
President François Hollande of France, addressing a rare joint session of Parliament in Versailles, urged lawmakers to extend for three months the state of emergency he declared after the attacks on Friday night. That designation allows the government to strip the citizenship of French natives who are convicted of terrorism and hold a second passport.
Declaring that “France is at war,” he said the attacks had been “planned in Syria, organized in Belgium, perpetrated on our soil with French complicity.”
He described Syria as “the biggest factory of terrorists the world has ever known,” and said France would continue the airstrikes it launched on Sunday night against Islamic State strongholds in Syria.
The French authorities said on Monday that they had conducted 168 raids across the country in an effort to root out possible terrorist threats. The raids extended from the Paris region to the major cities of Lille, Lyon, Marseille and Toulouse, they said. They also said they had arrested 23 people and detained 104 others under house arrest.
But a Frenchman believed to be involved in the Paris attacks, Salah Abdeslam, 26, a brother of Ibrahim Abdeslam, remained at large, eluding a series of raids conducted by the authorities in Molenbeek, the working-class Brussels neighborhood where the brothers lived.
A third brother, Mohamed, and four other men who had been detained in Belgium were released on Monday. At a news conference in Brussels, Mohamed said he did not know Salah’s whereabouts and added, “My parents are under shock and have not yet grasped what has happened.”
The alleged architect of the plot, Mr. Abaaoud, who traveled to Syria last year and even persuaded his 13-year-old brother to join him there, is from the same neighborhood, Molenbeek, as the Abdeslam brothers.
Mr. Abaaoud was already a suspect, according to officials and local news reports, in a failed terrorist plot in Belgium in January and an attempt in August to gun down passengers on a high-speed train to Paris from Brussels. The official said the authorities feared he might be in Europe.
At noon, France observed a moment of silence in honor of the victims of the attack, which killed 129 and wounded about 350 others. The Métro and cars stopped and crowds gathered at a makeshift memorial at the Place de la République and at the Eiffel Tower. Mr. Hollande stood with students at the Sorbonne. Many recited the national anthem, “La Marseillaise,” after the moment passed. In other cities — Delhi, Doha and Dublin — crowds gathered at French embassies to pay their respect.
As France observed its second of three days of national mourning, the authorities in both France and Belgium raced to track down suspects and chase leads.
At one house in the Rhône department in the southeast, around Lyon, the police found a Kalashnikov assault rifle, three automatic pistols, ammunition and bulletproof vests. Officers then obtained a warrant to search the home of the parents of a man who lived in the house, where they found several automatic pistols, ammunition, police armbands, military clothing and a rocket launcher.
Prime Minister Manuel Valls and Interior Minister Bernard Cazeneuve promised to keep up the search.
“We are using all the possibilities given to us by the state of emergency, that is to say administrative raids, 24 hours a day,” Mr. Valls said, vowing to keep intense pressure on “radical Islamism, Salafist groups, all those who preach hatred of the Republic.”
The authorities also confirmed on Monday that one of the attackers had entered Europe through Greece on a Syrian passport last month, posing as a migrant.
The man was identified on his passport — found at the soccer stadium north of Paris where he blew himself up on Friday night — as Ahmad al-Mohammad, 25, a native of Idlib, Syria. The holder of the passport passed through the Greek island of Leros on Oct. 3 and the Serbian border town of Presovo on Oct. 7, according to Greek and Serbian officials. It remained unclear if the passport was authentic.
All told, at least four French citizens were among the seven attackers. Ibrahim Abdeslam; Mr. Mostefaï, who met with the suspected planner of the attacks; and two men identified on Monday as Samy Amimour, 28, a Paris native who lived in the suburb of Drancy, and Bilal Hadfi, 20, who lived in Brussels.
Mr. Amimour was known to the French authorities, having been charged in October 2012 with terrorist conspiracy, according to the authorities. He was placed under judicial supervision but violated the terms of that supervision in the fall of 2013, prompting the authorities to put out an international arrest warrant.
Last December, the French newspaper Le Monde had interviewed Mr. Amimour’s father — it did not identify him by name at the time — who had gone to Syria to try to bring back his son. Three members of the Amimour family were detained on Monday.
The Turkish government confirmed on Monday that Mr. Mostefaï, 29, had entered Turkey in 2013 but said “there is no record of him leaving the country.”
A senior Turkish official, speaking on the condition of anonymity, said the government flagged Mr. Mostefaï twice — in December 2014 and again in June of this year — but that “we have, however, not heard back from France on the matter.” He added, “It was only after the Paris attacks that the Turkish authorities received an information request about Ismaël Omar Mostefaï from France.”
The official added that “this is not a time to play the blame game” but added that governments needed to do better at sharing intelligence to prevent terrorism.
The United States has provided logistical support for the French airstrikes in Syria, but President Obama on Monday again ruled out a ground intervention. “Let’s assume that we were to send 50,000 troops into Syria,” he said at a gathering of leaders of the Group of 20 industrial and emerging-market economies in Antalya, Turkey. “What happens when there’s a terrorist attack generated from Yemen? Do we then send troops into there? Or Libya, perhaps?”
Elsewhere in Europe, the authorities tightened security. Britain on Monday announced that it would pay for an additional 1,900 intelligence officers, and review aviation security, as part of its response to the attacks.
The home secretary, Theresa May, said there would be tighter surveillance of those arriving in Britain and that border guards were making targeted checks of passengers and vehicles leaving for France.
Prime Minister David Cameron, who like Mr. Obama attended the Group of 20 meeting, said he would consider speeding up the legislative timetable for a proposed law to govern electronic surveillance by the intelligence agencies, though he added that it was important to bring Parliament and public support with him.
In Washington, John O. Brennan, the director of the Central Intelligence Agency, said on Monday that both the Paris attacks and the crash of a Russian jet over the Sinai Peninsula bore the “hallmarks” of the Islamic State.
Speaking at the Center for Strategic and International Studies, Mr. Brennan called the group an “association of murderous sociopaths” that is “not going to content itself with violence inside the Syrian and Iraqi borders.”
Wading into the debate over surveillance, privacy and encryption, Mr. Brennan said he hoped the Paris attacks would be a “wake-up call,” adding “hand-wringing” had weakened the ability of Western intelligence services to prevent attacks.