PARIS • On both sides of the Atlantic, the fast-moving investigation into the deadly Paris terrorist attacks has accumulated clues: A car discovered in a Paris suburb with a cache of weapons; mounting proof of links with the Islamic State in Iraq and Syria (ISIS); 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 under way for a suspect described as dangerous and much still unknown, increasing evidence suggested that at least one of the eight attackers had visited Syria, where ISIS has its main stronghold.
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 American security services had alerted them in September to vague, but credible information, that French extremists 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 pre-emptive air strikes on Oct 8 against ISIS' self-declared capital in Syria, Raqqa.
The coordination of the latest attacks suggest a growing and ominous sophistication among terrorist networks, American and French officials said.
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.
A SENIOR EUROPEAN COUNTER-TERRORISM OFFICIAL, commenting on the attackers
The attacks also illustrated how such networks, operating in Europe, are oblivious to national boundaries. The authorities said several of the assailants had lived quietly in Belgium even as they prepared to strike in France.
European intelligence officials said the one attacker who they believed had gone to Syria was Ismael Omar Mostefai, a French citizen. He travelled 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.
"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 counter-terrorism official.
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.
But some analysts noted that many aspects of the assault also failed. The suicide bombers sent to attack the football match between France and Germany did not inflict many casualties. And ISIS had boasted of carrying out an attack in the 18th Arrondissement, an administrative district, but it never happened. Also, the suicide bombs used by at least six of the attackers were unsophisticated, according to some analysts. At one cafe in Paris, an attacker managed to kill only himself.
Even so, analysts and security officials agreed 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.