ISIS has claimed responsibility for the worst violence witnessed in France since World War II, a volley of nearly simultaneous terror attacks that the French President called "an act of war."
The assailants targeted six sites Friday night in Paris, the deadliest being a massacre at a concert hall where at least 80 people were killed.
In all, French authorities put the number of dead at 128, though the death toll is expected to fluctuate as the situation becomes clearer.
The threat of ISIS is well-known, with the jihadist group's atrocities in Syria and Iraq being met with condemnation and airstrikes by a U.S.-led coalition that includes France.
But the scale and apparent coordination of Friday's attacks inside the European Union, which comes on the heels of ISIS' claim of taking down a Russian airliner in Egypt, represent an escalation of capabilities if confirmed.
In an online statement distributed by supporters Saturday, ISIS said eight militants wearing explosive belts and armed with machine guns attacked precisely selected areas in the French capital.
A Syrian passport was found near the body of an attacker outside one of the targeted sites, the Stade de France, according to a police source, CNN affiliate France 2 and other French media reported.
In addition to those killed, 180 others were injured, according to the Paris Police Prefecture. More than half of them are in critical condition.
Americans are among the injured in the attacks, U.S. State Department deputy spokesman Mark Toner said Saturday.
President Francois Hollande blamed the attacks on ISIS, calling it "an act of war" by the militants. He said it was planned from the outside -- "with inside complicity."
"When the terrorists are capable of doing such acts, they must know that they will face a France very determined," he said.