Intelligence officials in the U.S. and Europe picked up “chatter” as early as September about a potential Islamic State-related attack on France, it emerged Monday, as evidence grew that some of the men involved in last week’s terrorist assault on this city – including its possible mastermind – were known to French police.

The intercepted communications flowed from Islamic State leaders in Syria to followers in Europe, among whom, apparently, were members of the Brussels-based cell French authorities now believe carried out Friday’s attacks, a U.S. law-enforcement official said.

The official spoke on condition of anonymity because of the delicacy of the investigation into the audacious string of bombings and shootings, which killed at least 129 people and wounded hundreds more.

Although no specific target or date was given in the intercepts for an attack, making it hard to distinguish the threat from similar signals picked up by intelligence agencies, France was eager to pursue the lead, the official said.


But the trail suddenly vanished because the militants switched in September from open communication sources to so-called PS4 embedded devices – such as Sony PlayStation 4 equipment – that use encryption and block authorities from tapping them.

“The French were trying to find out anything more about the chatter, and they got behind it,” the official said. “But it went bad.”

The revelation of intelligence that pointed to a possible attack came on a day of intensive police operations in Belgium and France to hunt down a suspect in last week’s massacre in Paris and to clamp down on dozens of other alleged extremists.

The French government vowed to mount an all-out response against Islamic State, whose base in northern Syria was the target of heavy airstrikes by French warplanes Sunday night. In a rare address to both houses of Parliament, President Francois Hollande declared that “France is at war” and asked for expanded powers for police and his office to combat the threat of terrorism.


But authorities here acknowledged that another of the seven dead suicide bombers had already been known to them before Friday’s attacks and had fallen out of their grasp.

Samy Amimour, a 28-year-old Frenchman, was charged in a terrorism investigation in 2012 after an unsuccessful attempt to travel to Yemen. He was put under judicial supervision but then disappeared from official radar in fall 2013, leading to an international warrant for his arrest.

Amimour was one of three suicide bombers Friday who roved around with automatic weapons inside the Bataclan concert hall, where 89 people were killed, the Paris prosecutor’s office said.

Another of the attackers who died at the theater, Ismael Omar Mostefai, had also been on the authorities’ watch list of potential militants, but not at a level requiring extensive surveillance.


The prosecutor’s office identified one of three bombers who detonated themselves outside the Stade de France sports arena as 25-year-old Ahmad Al Mohammad. A Syrian passport with his name was found near his body, and his fingerprints matched those of a presumed asylum seeker who arrived in Europe via Greece last month.

It remained unclear whether the name and passport were authentic. Many fake Syrian identification documents are in circulation.

Much of the attention Monday began focusing on Abdelhamid Abaaoud, a Belgian acolyte of Islamic State widely named in media reports as the likely chief plotter of Friday’s attacks. Abaaoud also was known to authorities and was reportedly a suspect in a failed plot in Belgium in January.

The daily newspaper Le Monde reported that Abaaoud was also the suspected mastermind of a failed operation in August. In that plot, Reda Hame, a French militant, told authorities that Abaaoud had given him an encrypted USB flash drive, about $2,100 and instructions to pick a target, such as a “concert venue,” that would maximize the casualty count.


Eric van der Sypt, a spokesman for the Belgian federal prosecutor’s office, confirmed that Abaaoud and 26-year-old Salah Abdeslam, the suspect who was the focus of Monday’s manhunt, were acquainted with each other. French police have issued an arrest warrant for Abdeslam and described him as a dangerous suspect wanted in connection with Friday’s attacks. According to media reports, Abdeslam avoided arrest and slipped across the border into Belgium on Saturday.

Armed officers in balaclavas shut down part of the Molenbeek St. Jean district, known as an Islamist hotbed, in an attempt to find Abdeslam. But Van der Sypt said no arrests had been made.

Two men out of seven people who were arrested in Belgium over the weekend have now been accused of being members of a terrorist group and participating in a terrorist attack, Van der Sypt said. The other five were released, including Abdeslam’s brother Mohammad.

“We don’t know where my brother is right now. And with all these tensions, we don’t know if he’ll dare to surrender to justice or not,” Mohammad Abdeslam said.


A third brother, Brahim, has been identified in media reports as one of the suicide bombers, who blew himself up outside a Paris eatery but did not kill anyone else.

At least one car apparently used in the attacks was also linked to Molenbeek, which sits across an industrial canal from downtown Brussels.

The unflattering spotlight on their multicultural enclave has angered many of its residents. The neighborhood is home to a large number of immigrants, especially from Morocco.

“The real people of Molenbeek are like me, hard-working people working to care for their families,” said Ismael Ahadouch, 26, dressed in a black track suit. “We are as much against these terrorists as anyone. We are educated people here, not fanatics.”


In France, police fanned out across the country overnight and raided 168 locations, using powers granted under the government-declared state of emergency. Interior Minister Bernard Cazeneuve said that 104 people had been placed under house arrest.

The raids were part of a national crackdown on extremism, but most were not directly connected to Friday’s attacks.

“It’s just a start,” Cazeneuve said. “The one who targets the republic, the republic will catch him, will be implacable.”

Hollande, the French president, said in his special address that he would seek to extend the state of emergency by three months.


Speaking in the opulent Palace of Versailles, he also asked lawmakers to amend the constitution to give his office more power when the country is faced with an immediate, serious threat, and to allow the government to strip French citizenship from people with dual nationality who are convicted of terrorism or “threatening the nation’s interests.”

Hollande also promised more airstrikes against Islamic State’s headquarters in Syria. He called for a special meeting of the United Nations Security Council.

“We are not engaged in a war of civilizations. These murderers do not represent any civilization,” he said. “Jihadist terrorism threatens the whole world, and not only France.”

Friday’s attacks were “acts of war [that] were decided and planned in Syria. They were organized in Belgium and perpetrated on our soil, with French complicity,” Hollande said. “It’s cruel to say: It’s French people who on Friday killed other French people.”


The country observed a minute of silence at noon Monday, the second time in less than a year that France has stopped to commemorate victims of a lethal terrorist attack in Paris. The first was in January, after the killing of 17 people by Islamist militants at the offices of the satirical magazine Charlie Hebdo, a kosher supermarket and other sites.

Chu reported from Paris and Serrano from Washington. Times staff writer Patrick J. McDonnell in Brussels contributed to this report.

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