PARIS — Three teams of killers carried out the coordinated suicide mission across Paris on Friday that killed 129 people and wounded at least 352 more, French officials said Saturday.
French troops policed the streets in the aftermath as investigators scoured six bloody crime scenes and shock hovered over the city after the deadliest attack on French soil since Nazis invaded in World War II.
French President Francois Hollande vowed a "merciless" response to the slaughter after ISIS claimed responsibility.
The assailants, working in three teams, attacked concert-goers, cafe diners and soccer fans in at least six locations in the French capital. Paris prosecutor Francois Molins said that seven attackers were killed in the carnage, including six who blew themselves up with suicide belts. Authorities earlier had said eight attackers were involved.
French media partially identified two of the attackers as Ismail M. and Abbdul Akbak B. A former French official confirmed to NBC News that those partial identities are correct. The brother and father of the attacker identified as Ismail M. were taken into custody Saturday, the official said.
One American, a college student from California, has been identified among those killed. Nohemi Gonzalez, 23, a junior at Long Beach State, was spending a semester studying in Paris. The U.S. State Department said other Americans were among the wounded.
Belgium's justice minister said Saturday that several arrests connected to the attacks were made in Brussels, the Associated Press reported. Minister Koen Geens told the VRT news outlet in Belgium that the arrests were connected to a rental car with Belgian license plates that was seen close to the Bataclan theater, one of the Paris sites where victims were killed.
Speaking early Saturday, Hollande pointed the finger at ISIS for orchestrating the "cowardly" attacks which he described as an "act of war committed by a terrorist army" and organized from abroad. He said France remained "unbreakable," calling for "unity and courage."
Hollande added: "We will work alongside our allies to fight this terrorist menace ... France is strong and even if she is wounded she will get up always and nothing will hold her down, even if we are feeling the grief now ... We will defend ourselves."
Earlier, Hollande vowed to be "merciless" with the nation's foes.
A statement issued by ISIS later claimed responsibility for the attacks, according to global security firm and NBC News analyst Flashpoint Intelligence. ISIS has previously threatened France due to its military operations against the group in Syria and Iraq.
"We are at war," France Prime Minister Manuel Valls tweeted. "And this war, we will win."
ISIS released an undated video Saturday urging Muslims who are unable to wage holy war in Syria to carry out attacks in France, according to an unclassified FBI document.
"Indeed, you have been ordered to fight the infidel wherever you find him," an unidentified ISIS militant says in the video. "What are you waiting for? There are weapons and cars available, and targets ready to be hit."
A French official close to the investigation confirmed to NBC News that a Syrian passport was found on one of the attacker's bodies. The passport holder entered the European Union through Greece last month, a Greek official said Saturday.
It was not immediately clear whether the attacker was the passport holder.
The violence came less than a year after deadly attacks on satirical magazine Charlie Hebdo and a kosher supermarket drove home the threat of Islamist terrorism here.
"This time it's war," pronounced Saturday's front page of Le Parisien newspaper.
All museums, schools and libraries in Paris were ordered closed Saturday as investigators searched for clues following the bloodshed. The Eiffel Tower, Louvre museum and nearby Disneyland theme park were also shut.
The Bataclan concert hall in the lively 11th arrondissement was the scene of the night's worst carnage. Dozens of people died there when gunmen opened fire during a sell-out concert by American band Eagles of Death Metal.
The attackers held hundreds of people hostage before blowing themselves up. Footage obtained by Le Monde showed concert-goers hanging from venue's third-floor windows while others ran for their lives out a rear exit.
French police stormed the venue after midnight, killing one assailant while two others detonated suicide belts. The wounded, many in critical condition, were rushed to waiting ambulances as sirens wailed.
Sylvain, a 38-year-old concert-goer, collapsed in tears as he recounted the attack to the AP.
"There were shots everywhere, in waves. I lay down on the floor. I saw at least two shooters, but I heard others talk. They cried, 'It's Hollande's fault.' I heard one of the shooters shout, 'Allahu Akbar' ['God is greatest']," he said, speaking on condition that his full name not be used out of concern for his safety.
Several neighborhood bars were transformed into trauma units to treat and receive survivors.
Maurice Mellul, 24, came to lay flowers outside the Bataclan on Saturday morning.
"I'm very sad," he told NBC News. "I have a lot of rage and hatred. We need to continue our lives, we can't stay inside. We must continue, despite tragic events, we must continue our lives to fight this."
The siege of the concert hall came around the same time that three suicide bombers targeted spots near the national soccer stadium as the France played an exhibition game against Germany.
Karl Olive, the mayor of Poissy, was among thousands of spectators at the Stade de France watching the game. He told NBC News he heard an explosion — "it didn't sound like a firecracker, it sounded like a bomb" — but the match continued.
Hollande was also attending the game but was rushed away to deal with the situation. Fans later sang the country's national anthem as they filed out of the stadium.
Meanwhile, gunmen opened fire on diners at a string of cafes in a trendy neighborhood, which were crowded on an unusually balmy November night.
French officials shut down the subway and ordered people to remain indoors as the events unfolded, beginning just before 9:30 p.m. local time (3:30 p.m. ET). The killers, struck five locations after the first bomb exploded at the stadium.
Residents were out in the streets as normal on Saturday.
Parisians offered shelter to anyone caught up in or stranded in the chaos, setting Twitter alight with the hashtag #porteouverte — or "open door." Americans echoed the charity with #StrandedInUS.
Users of Facebook shaded their profile pictures in the French Tricolor, and on Twitter and Instagram, people posted vacation photos, teardrops and a peace symbol with the Eiffel Tower inscribed in the center as they expressed their grief over the carnage.
Notre Dame Cathedral was scheduled to hold a memorial service for the victims of the attacks on Sunday.
President Barack Obama said the violence was an attack on "all of humanity"while Secretary of Defense Ash Carter called it an "assault on our common human dignity."
While federal and local officials said there was no credible or specific intelligence about threats to the U.S., law enforcement agencies were on alert and deployed extra patrols following the Paris attacks.
The attacks Friday hit a nation still reeling from January's three-day terror spree that left 20 people dead — including three attackers, who opened fire at the offices of Charlie Hebdo magazine and later laid siege on a kosher supermarket.
Bataclan — the nightclub central to Friday's carnage — is situated not far from Charle Hebdo's offices.
France has seen several smaller-scale attacks or attempts this year, including an incident on a high-speed train in August in which American travelers thwarted an attempted attack by a heavily armed man.
French authorities are particularly concerned about the threat from hundreds of French Islamist radicals who have traveled to Syria and returned home with skills to stage violence.
Sarah Burke, Emmanuelle Saliba, Matthew Grimson, Jake Cigainero, Abigail Williams , The Associated Press and Reuters contributed. This article appeared on NBCNews.com.