PARIS — On both sides of the Atlantic, the fast-moving investigation into the deadly Paris terrorist attacks steadily accumulated clues Sunday: a car discovered in the Parisian suburbs with a cache of weapons. Mounting proof of links between the Islamic State in Syria and the attackers. And intense scrutiny on three brothers, living in Belgium, as crucial suspects in the elaborate plot.
With investigators moving on multiple fronts and a manhunt underway for a suspect described as dangerous, with much still unknown, increasing evidence suggested that at least one of the eight attackers had visited Syria, where the Islamic State has its main stronghold.
Others had been communicating with known members of the group before the horrific assault on Paris, investigators said. Officials were also investigating the possibility that a Syrian citizen may have been sent to join the attackers, slipping into Europe along with thousands of refugees.
French officials said US security services had alerted them in September to vague but credible information that French jihadis in Syria were planning some type of attack.
That tip, the officials said, contributed to France's decision to launch what it had hoped might be preemptive airstrikes on Oct. 8 against the Islamic State's self-declared capital in Syria, Raqqa, where France struck with a new and far larger round of airstrikes Sunday night — this time in retaliation.
The attacks also illustrated how terrorist networks operating in Europe are oblivious to national boundaries, posing yet another challenge. The authorities said several of the assailants had lived quietly in Belgium even as they prepared to strike France.
European intelligence officials said that the one attacker who they believed had gone to Syria was Ismael Omar Mostefai, a French citizen. He traveled to Turkey in 2012, and probably then slipped into Syria.
European officials said they believed the Paris attackers had used some kind of encrypted communication, but offered no evidence. "The working assumption is that these guys were very security aware, and they assumed they would be under some level of observation, and acted accordingly," said a senior European counterterrorism official who spoke on the condition of anonymity to discuss confidential information.
A French official said that some of the attackers had shown a discipline that suggested military-style training, and that the plot involved considerable planning and input from an organized group.
But some analysts noted that many aspects of the assault had failed. The suicide bombers sent to attack the soccer match between France and Germany at the Stade de France did not inflict many casualties.
And the suicide bombs used by at least six of the attackers were unsophisticated, according to some analysts.
Even so, analysts and security officials agreed that the willingness of the attackers to carry out suicide bombings and to kill relentlessly with assault rifles suggested a new level of commitment for attacks in Europe.
"It is coordinated," said Alain Bauer, a French criminologist who serves on an advisory council to the government. "But the big thing is the determination of the attackers. That in France and Europe is new — people who are willing to kill bullet by bullet."
The French interior minister, Bernard Cazeneuve, said the attackers had "prepared abroad and had mobilized a team of participants in Belgium, and who may have benefited — the investigation will tell us more — from complicity in France."
The immediate challenge for investigators is to identify all the attackers and piece together how they carried out the plot. French officials say that six attackers died by suicide bombs and a seventh died in a shootout with the police.
One who died after setting off a suicide bomb at the Bataclan concert hall, where 89 people were killed, was identified by the authorities after they recovered a finger and matched his prints to a file that listed him as radicalized and a potential security threat. He was named as Mostefai, a native of Courcouronnes, France.
In the effort to trace the others, investigators focused on Belgium. Authorities there have arrested several people in Molenbeek, a poor section of Brussels that is home to many Arab immigrants that has been linked to past terrorist attacks.