French leaders vowed Monday to hunt down Islamic State militants behind last week’s attacks in Paris as European authorities intensified efforts to untangle a plot that they believe leads all the way back to Syria.

President François Hollande, in his first address to French lawmakers at Versailles — assembled in a joint session of the two chambers — promised an unforgiving campaign against the Islamic State and proposed changes to France’s constitution to help authorities beat back militant threats.

“It is not about containing but about destroying that organization,” Hollande said before the members of Parliament stood to sing the national anthem. “They are not out of our reach.”

Hollande spoke as European authorities expanded the manhunt for suspects involved in Friday night’s violence, which killed 129 people in a series of coordinated assaults.

By late Monday, French and Belgian officials had conducted more than 160 raids, arrested more than 20 suspects and seized weapons as they sought to identify others involved in planning the attacks and pinpoint links between attackers and the Islamic State’s leaders in Syria and Iraq.

Remembering the victims of Friday’s attacks in Paris

Authorities were zeroing in on the role of a man they believe is a key figure in the Islamic State’s operations in Europe and possibly played a coordinating role in the Paris plot from Syria.

Abdelhamid Abaaoud, a 28-year-old Belgian national of Moroccan descent, has already been linked to a number of terrorist attempts in Europe this year, including a foiled assault aboard a high-speed Paris-bound train in August.

A French official familiar with the case described Abaaoud as the “guru” of several assailants, including Salah Abdeslam, a 26-year-old believed to have taken part in Friday’s bloodshed who is now the subject of an international dragnet.

While the expanding investigation produced tantalizing clues about possible plotters, it also underscored the limitations of Western security agencies as they face homegrown terrorism plots.

New information about the attackers showed that at least some were known to French and Belgian security officials. Turkish and Iraqi officials also reported having warned Western officials about potential threats ahead of Friday’s attacks.

Manuel Valls, the French prime minister, said the attack was “organized, conceived and planned” from Syria. Waves of migrants fleeing the civil war there have traveled to Europe, raising worries that militants could also have used the exodus as way into the continent.

What we know so far about who carried out the Paris attacks

Rep. Adam B. Schiff (Calif.), the ranking Democrat on the House Intelligence Committee, also said emerging intelligence about the plot suggests that it originated in Syria. “It certainly looks as if the general plotting originated in Syria,” Schiff said in Washington, adding that it remains unclear “to what degree operatives in Europe may have been exercising their own discretion in choice of timing and targets.”

Schiff spoke after he and other lawmakers received a briefing Monday. He declined to elaborate, but said the Islamic State “has been trying to plant operatives in Europe for some time, building up their external operations, and France has been a primary target.”

In Brussels on Monday, dozens of Belgian police officers sealed off a street in the largely Muslim Molenbeek district and conducted a raid there, but they failed to catch the fugitive they were seeking.

Magnus Ranstorp, research director at the Center for Asymmetric Threat Studies at the Swedish Defense University, said the free flow of people through the European Union has made it difficult for underfunded intelligence agencies in France and Belgium, which has emerged as a nexus for Islamic State supporters, to track potential threats.

In Washington, CIA Director John Brennan rejected the idea that the attacks reflected an intelligence failure. He blamed leaks about surveillance capabilities for undermining the ability of spy agencies to protect people. “Clearly there was an effort that was underway for quite some time,” Brennan said of the Paris plot. He said European security services’ “ability to monitor and surveil these individuals is under strain.”

The United States and its allies knew the Islamic State was planning and threatening attacks in Europe, he said, but “there has been a significant increase in the operational security of these operatives and terrorist networks as they have gone to school” on disclosures of U.S. and allied capabilities. He did not name Edward Snowden, the former U.S. intelligence contractor who leaked details about U.S. eavesdropping programs, but lashed out at “unauthorized disclosures and a lot of hand-wringing over the government’s role.”

The potential for violence elsewhere in the West was highlighted Monday when a purported Islamic State subgroup released a video showing militants warning that Washington could be next.

Speaking at a Group of 20 summit in Antalya, Turkey, President Obama said a new agreement to expand intelligence-sharing would help the United States more quickly provide France information about threats from groups such as the Islamic State, also known as ISIS and ISIL.

“The attacks in Paris remind us that it will not be enough to defeat ISIL in Syria and Iraq alone,” Obama told reporters.

He characterized the violence as a “terrible and sickening setback” but said it did not detract from progress being made by the United States and its allies in Iraq and Syria.

U.S. Secretary of State John F. Kerry, who arrived in Paris on a hastily arranged visit, expressed American condolences.

“Don’t mistake what this represents,” he said of the fight against the Islamic State. “This is not a clash of civilizations.” These “terrorists,” he said, have attacked “all civilizations. There is nothing, nothing civilized about them.”

Hollande is expected to put forward a bill this week to extend a state of emergency for three months, enhancing police power to restrict freedom of movement and gatherings at public places.

At Versailles, he also proposed constitutional changes that would allow authorities to withdraw French citizenship from people with dual nationality, even if they were born in France, and to prevent French terrorism suspects from returning to France.

Hollande said he would meet with Russian President Vladimir Putin and Obama in coming days.

In France, the nation observed a moment of silence. The Eiffel Tower, which dimmed its lights in mourning, was re-illuminated at sundown in the national colors of blue, white and red.

In Belgium, the federal prosecutor said the government would charge two individuals seized over the weekend with terrorism but released five others detained late Saturday and early Sunday.

French police initially said that eight assailants took part in the Paris attacks in three groups — with seven dying amid the bloodshed. The possibility that an eighth attacker, thought to be ­Abdeslam, was still at large raised hope he could be captured alive and provide critical information on how the attacks took shape and were funded and directed.

“Let this be clear to everyone,” French Interior Minister Bernard Cazeneuve said as raids were carried out across the country. “This is just the beginning. These actions are going to continue.” He said at least 23 people were detained in overnight raids — at least three near the southern city of Toulouse and several near Lyon — and weapons were seized, including a rocket launcher and automatic rifles.

Authorities also identified two more of the attackers, one of them a 28-year-old Frenchman already charged in a terrorism investigation in 2012. Samy Amimour, who blew himself up at the Bataclan music hall Friday night, had been placed under judicial supervision. An international arrest warrant was issued in fall 2013 after he failed to comply with bail conditions. Three of his relatives were placed under police custody Monday morning.

The other new name was that of Ahmad al-Mohammad, who detonated his explosives outside the national soccer stadium. He was found with a Syrian passport that identified him as Ahmad Almohammad, a 25-year-old born in Idlib. The prosecutor’s office says fingerprints from the attacker match those of someone who passed through Greece in early October. The passport may not have been authentic.

Mufson reported from Brussels. Karen DeYoung, Souad Mekhennet, Cléophée Demoustier, Virgile Demoustier, Karla Adam and Monique El-Faizy in Paris, Liz Sly in Baghdad, Hugh Naylor in Beirut, Brian Murphy, Greg Miller and William Branigin in Washington and Elinda Labropoulou in Athens contributed to this report.

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