By Mariano Castillo, Margot Haddad, Michael Martinez and Steve Almasy
PARIS (CNN) -- Authorities in France and Belgium took people into custody Saturday in the aftermath of the worst violence in France since World War II, a series of terror attacks that killed more than 120 people.
ISIS claimed responsibility for the horrific Paris attacks, for which France vowed revenge.
President Francois Hollande deemed the shootings and bombings "an act of war." He said early Saturday, "We will lead the fight, and we will be ruthless."
Belgian authorities made a number of arrests there in the first publicized apprehensions after Friday night's bloodshed, a Belgian Justice Ministry spokeswoman said Saturday. In Belgium, raids were conducted in the Brussels suburb of Molenbeek-Saint-Jean, Justice Ministry spokeswoman Sieghild Lacoere said. A car rented in Brussels was found near one of the sites of the Paris attacks, and "that's what triggered the raids," Lacoere said.
In all, the raids took place in three homes in Molenbeek-Saint-Jean, a Western intelligence source told CNN.
At least one of the raids is connected to the Paris attacks, according to the source, who is in contact with French and Belgian intelligence services. The other raids are connected to individuals known to Belgium intelligence, the source said. Some of the Paris attackers are also known to Belgium intelligence, the source added.
Also Saturday, CNN affiliate BFMTV reported that the father and a brother of one of the attackers had been taken into custody. And AFP reported that the two men were detained after police raided their homes 130 kilometers (81 miles) east of Paris. CNN has not independently verified that the men were picked up by authorities.
One of the suicide bombers in Friday's attacks has been identified as Ismael Omar Mostefai, according to a French member of Parliament. Mostefai lived in Chartres at least until 2012, said Jean-Pierre Gorges, who is mayor of the French town as well as a member of Parliament, via Facebook.
Man stopped at border
A man who rented a VW Polo used by terrorists at the Bataclan concert venue was intercepted at the border with Belgium, Paris prosecutor Francois Molins said. The man, who was driving a different vehicle when he was caught, is a French national living in Belgium and was accompanied by two other people, Molins said.
One of the terrorists who died in Paris was identified as a 29-year-old French national from Courcouronnes in the city's southern suburbs, Molins said Saturday.
That individual was involved in the attack on a concert hall, had a criminal history and was identified as having been radicalized in 2010, but that person had never been accused of terrorism, Molins said.
Molins said the attacker was identified by fingerprints.
In the nearly simultaneous attacks on Friday night, the assailants targeted six sites, the deadliest being a massacre at a concert hall where at least 80 people were killed.
In addition to the 129 people killed in Paris, 352 were injured, at least 99 seriously, Molins said Saturday. Seven terrorists were killed, French officials have said. They all had assault rifles, Molins added.
At least one American is among the 129 dead, officials said. The U.S. victim was Nohemi Gonzalez, 23, of El Monte, California, a junior studying design in Paris for a semester while enrolled at California State University, Long Beach, the school said.
Three Chileans also were killed, as were two people from Belgium, two from Mexico, two from Spain, one from Portugal, one from the United Kingdom and many French citizens. Other nations whose citizens were killed had yet to identify those victims.
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.
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.
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.
The passport belonged to a person who had been processed on the Greek island of Leros, Greek Deputy Minister of Citizen Protection Nikos Toskas said Saturday. CNN cannot independently verify that the passport was authentic or whether it actually belonged to one of the attackers.
A source close to the investigation told CNN that an Egyptian passport was found on another attacker. "There is strong assumption that these passports are fake," the source said.
Hollande blamed the attacks on ISIS and said they were 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.
While ISIS' claims have not been confirmed, a senior U.S. intelligence official told CNN the U.S. government has "no reason to doubt" Hollande's attribution of the attacks to the terrorist group.
The coordination and sophistication of such attacks are the most recent evidence that ISIS is eclipsing al Qaeda as the most significant global terrorist threat.
Response in wake of attacks
Hollande issued a state of emergency and called for three days of mourning after the attacks unfolded.
On Saturday, French Interior Minister Bernard Cazeneuve elaborated that the state of emergency could mean restrictions on people's movements. Border controls were tightened as of Friday, and the gendarmerie paramilitary police are on heightened alert, he said.
France has beefed up security forces at public transportation hubs, on the main roads and highways as well as everywhere in the center of Paris, Cazeneuve said after a meeting with Hollande.
Night of horror
Gunmen hit Friday night when bars and restaurants were bustling with residents and tourists. When they stormed in, glass shattered under the rage of bullets. Excited weekend chatter turned into panicked screams.
One of the targets was near a soccer match as France played world champion Germany. Terrified fans huddled together and streamed onto the field after blasts went off. Others hugged.
At the Bataclan, a concert hall where most of the fatalities occurred, fans were listening to American rock band Eagles of Death Metal when the blasts started.
"People yelled, screamed," said Julien Pearce, a radio reporter who was there. "It lasted for 10 minutes. Ten horrific minutes where everybody was on the floor covering their head."
At least 80 people died there.
Other attacks took place along streets in Paris and at the national stadium in Saint-Denis.
CNN iReporter Chris Morrow of San Diego was staying five minutes from Le Petit Cambodge, a Cambodian restaurant covering an event.
"I heard an explosion and pop, pop, pop, which sounded like gunfire. I instinctively grabbed my phone and started filming out of the window."
"The men were huddling around, and there was no one on the street except them. They had long shotguns and seemed hysterical, screaming and not really sure what to do.
"They tried a few apartment doors, trying to get in, but they were locked. In the end, they ran toward the river. I am not sure if they were police or the attackers."
Solidarity and fear
There has been an outpouring of support and solidarity in Paris, but large gatherings in the streets have not materialized in part because the state of emergency prohibits those.
But there's more to it than those restrictions, Paris Deputy Mayor Patrick Klugman said.
"There is more sadness and probably more fear," Klugman told CNN's Christiane Amanpour. "We don't know yet if this terror in fact is over or not. And, from time to time on social media, you have more reports -- most (of it) false news -- but it shows how worried" the population is.
In the aftermath of the attacks, Parisians helped each other out by using social media to invite those in need inside their homes.
The message spread and "all around here, people opened up the doors of their buildings of their flats to welcome people," Klugman said.
President Barack Obama spoke to Hollande and offered him support and condolences. The FBI and the Department of Homeland Security are closely monitoring the situation, but there is no credible or specific threat in the United States, a U.S. government official said.
German Chancellor Angela Merkel said the attackers hate freedom and vowed that her nation will help lead the fight against terrorists.
Pope Francis on Saturday called the attacks part of the "piecemeal Third World War."
"There is no religious or human justification for it," he said in a telephone interview with TV2000, the television network of the Italian Bishops' Conference.
"I am close to the people of France, to the families of the victims, and I am praying for all of them," Pope Francis said in the interview, according to a statement from the Vatican. "I am moved, and I am saddened. I do not understand. These things (are) hard to understand."
In January, two brothers attacked the Paris offices of the French satirical magazine Charlie Hebdo, killing 12 and wounding 11.
Two days later, they were shot to death in a standoff with police in Dammartin-en-Goele.
Their associate attacked a Jewish grocery store in Paris, taking more than a dozen people hostage and killing four. He was killed by police.
Although the country has been wounded again, "France always rises up," Hollande said Saturday in the wake of the attacks.
CNN's Margot Haddad reported from Paris, and Mariano Castillo and Steve Almasy wrote from Atlanta. CNN's Faith Karimi, Michael Martinez, Richard Greene, Pierre Meilhan, Stephanie Busari, Pierre Buet, Pat St.Claire, Deborah Feyerick, Alanne Orjoux, Tim Lister, Peter Bergen, Chris Liakos and Pamela Brown contributed to this report.