PARIS — Seven people were detained Sunday in Belgium in connection with deadly attacks in Paris as the city entered three days of mourning for the 129 people killed in the worst violence in France in decades.

Thousands of French troops were deployed and tourist sites stood shuttered in one of the most visited cities on Earth, while more details started to emerge about the investigation.

The Islamic State group claimed responsibility for Friday’s gun and bomb attacks on a stadium, a concert hall and Paris cafes that also wounded 350 people, 99 of them seriously.

Three of the seven suicide bombers who died in the attacks were French citizens, as was at least one of the men arrested in neighboring Belgium suspected of links to the bloodshed.

A French police official confirmed that a suicide attacker identified by a skin sample was living in the Paris suburbs at the time of his death. A Belgian official said two of the seven people wired with suicide vests were French men living in Brussels, and among those arrested was another French citizen living in the Belgian capital.

The new information highlighted growing fears of homegrown terrorism in a country that has exported more jihadis than any other in Europe. All three gunmen in the January attacks on the Charlie Hebdo newspaper and a kosher supermarket were French.

Authorities said three teams of attackers were involved and seven suicide bombers blew themselves up — three near the stadium and three at the concert hall and one not far from it.

Authorities have not said if there are more attackers at large.

A French police official said Sunday that three Kalashnikovs were found inside a Seat car found in Montreuil, a suburb 6 kilometers (nearly 4 miles) east of the French capital. It was one of two vehicles known to have been used in the attacks. The official, who could not be named because the investigation is ongoing, said the weapons have not yet been analyzed.

In Belgium, an official said the seven people detained would hear later Sunday whether they would be held in custody longer. Three other people were arrested there Saturday.

The official, who spoke on condition of anonymity due to the ongoing investigation, also said two of the seven attackers who died in Paris on Friday night were French men living in Brussels. He said one was living in the Molenbeek neighborhood, which is considered a focal point for religious extremism and fighters going to Syria.

A Brussels parking ticket had been found inside the Volkswagen Polo parked outside the Bataclan concert hall, leading investigators to one of the men ultimately arrested in Belgium, according to a French police official who spoke on condition of anonymity because the investigation was ongoing.

Security was heightened across France, across Europe’s normally open borders, even across the ocean to New York, and how to respond to the Paris attacks became a key point among U.S. Democratic presidential hopefuls at a debate Saturday night.

President Barack Obama on Sunday called the terror attacks in Paris an “attack on the civilized world.”

Obama, speaking at the G-20 summit in Turkey focusing on fighting terrorism, pledged U.S. solidarity with France in the effort to hunt down the perpetrators and bring them to justice.

At the request of France, the European Union will hold a special meeting of its interior and justice ministers Friday to assess the impact of the Paris attacks.

In Paris, the shining sun and warm air felt cruelly incongruous.

Streets, parks and commerce were unusually empty for such a mild, clear day, and several city monuments were closed for security reasons or to express the city’s grief.

Some Parisians and tourists defied the high security, walking past heavily-armed soldiers in body armor to take pictures beneath the Eiffel Tower.

In its statement claiming responsibility, the Islamic State group called Paris “the capital of prostitution and obscenity” and mocked France’s air attacks on suspected IS targets in Syria and Iraq.

A French survivor of the rampage at Paris’ Bataclan concert hall said he was struck by how young the attackers were.

Julien Pearce, journalist at Europe 1 radio, was at the Bataclan concert hall on Friday to attend the concert by the American rock band Eagles of Death Metal. He said when the three attackers stormed in “it took me few seconds to realize it was gunshots.”

Pearce and his friends immediately got down on the ground to avoid the random shots, then ran and crawled into a tiny dark room next to the stage.

“There was no exit, so we were just in another trap, less exposed, but still a trap,” he said.

Pearce said he could discreetly look out and see one of the assailants. He says “he seemed very young. That’s what struck me, his childish face, very determined, cold, calm, frightening.”

Once the attackers reloaded, his group ran across the stage to the emergency exit, helping a wounded woman out. Looking back, he saw “dozens and dozens of entangled, bullet-riddled bodies in a pool of blood.” Eighty-nine people were killed at the hall.

President Francois Hollande has said that France, which is already bombing Islamic State targets in Syria and Iraq in a U.S.-led coalition, would increase its military efforts to crush IS and be “merciless” against the extremists.

The investigation sprawled well beyond France’s borders, since Paris Prosecutor Francois Molins said some attackers mentioned Syria and Iraq.

French authorities are particularly concerned about the threat from hundreds of French Islamic radicals who have traveled to Syria and returned home, possibly with dangerous skills.

Details about one attacker began to emerge: 29-year-old Frenchman Ismael Mostefai, who had a record of petty crime and had been flagged in 2010 for ties to Islamic radicalism. He was identified from fingerprints found on a finger amid the carnage from a Paris concert hall, the Paris prosecutor said. A judicial official and lawmaker Jean-Pierre Gorges confirmed his identity.

Police detained his father, a brother and other relatives Saturday night, and they were still being questioned Sunday, the judicial official said.

Struggling to keep his country calm and united after an exceptionally violent year, Hollande met Sunday with opposition leaders — conservative rival and former President Nicolas Sarkozy as well as increasingly popular far-right leader Marine Le Pen, who has used the attacks on Paris to advance her anti-immigrant agenda.

After the meeting, Sarkozy called for a change in France’s Syria policy, suggesting the country needs to work with both Russia and the U.S. to “destroy” IS.

“We need everyone … there can’t be two coalitions in Syria,” Sarkozy said.

On the streets, the entire nation was enveloped in mourning. Flags were lowered and Notre Dame Cathedral — closed to tourists like many Paris sites — planned a special church service later Sunday for victims’ families. Well-wishers heaped flowers and notes on a monument to the dead in the neighborhood where attackers sprayed gunfire on cafe diners and concert-goers.

Quentin Bongard said he left one of the targeted cafes after a fight with his girlfriend just moments before the attacks.

“Those are all places that I go often to,” the Paris resident said, still shaken. “We just want to come here, bring flowers, because we don’t want to be terrorized … but it is frightening.”

Even in their grief, residents were defiant about the lifestyle that has made their city a world treasure. Olivier Bas was among several hundred who gathered late Saturday at the site of the Bataclan hall massacre. Although Paris was quiet and jittery, Bas intended to go out for a drink — “to show that they won’t win.”

Meanwhile, refugees fleeing to the continent by the tens of thousands feared that the Paris attacks will prompt EU nations to put up even more razor-wire border fences and other obstacles to their quest to start a new life.

A Syrian passport found next to the body of one of the men who attacked France’s national stadium suggested that its owner passed through Greece into the European Union and on through Macedonia and Serbia last month. It’s a route tens of thousands of people fleeing war and poverty in Syria and elsewhere have been using.

A top European Union official insisted Sunday that the bloc’s refugee policy doesn’t need to be overhauled in the wake of the Paris attacks and urged world leaders not to start treating asylum-seekers as terrorists.

“Those who organized these attacks, and those who carried them out, are exactly those who the refugees are fleeing,” European Commission President Jean-Claude Juncker told reporters at the G-20 summit in Turkey. “There is no need to revise the European Union’s entire refugee policy.”