PARIS — On both sides of the Atlantic, the fast-moving investigation into the deadly Paris terrorist attacks steadily accumulated clues on 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 American security services had alerted them in September to vague but credible information that French jihadists in Syria were planning some type of attack.

Image Salah Abdeslam Credit... Police Nationale

That tip, the officials said, contributed to France’s decision to launch what it had hoped might be pre-emptive 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 carnage from the attacks in Paris, which so far have claimed 129 lives and left hundreds wounded, has presented France with its second major security breakdown in less than a year, after the terrorist assault in January against the satirical newspaper Charlie Hebdo and a kosher grocery store.

But the complexity and coordination of the latest attacks suggest a growing and ominous sophistication among terrorist networks, American and French officials said.

The attacks also illustrated how such networks operating in Europe are oblivious to national boundaries, posing yet another challenge. The authorities said that 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 Ismaël Omar Mostefaï, a French citizen. He traveled to Turkey in 2012, and probably then slipped into Syria.