France launched "massive" air strikes on ISIS's de-facto capital in Syria Sunday night, destroying a jihadi training camp and a munitions dump in the city of Raqqa, where Iraqi intelligence officials say the attacks on Paris were planned.

Twelve aircraft including 10 fighter jets dropped a total of 20 bombs in the biggest air strikes since France extended its bombing campaign against the extremist group to Syria in September, a Defence Ministry statement said. The jets launched from sites in Jordan and the Persian Gulf, in co-ordination with U.S. forces.

Meanwhile, as police announced seven arrests and hunted for more members of the sleeper cell that carried out the Paris attacks that killed 129 people, French officials revealed to The Associated Press that several key suspects had been stopped and released by police after the attack.

The arrest warrant for Salah Abdeslam, a 26-year-old born in Brussels, calls him very dangerous and warns people not to intervene if they see him.

Yet police already had him in their grasp early Saturday, when they stopped a car carrying three men near the Belgian border. By then, hours had passed since authorities identified Abdeslam as the renter of a Volkswagen Polo that carried hostage takers to the Paris theatre where so many died.

Three French police officials and a top French security official confirmed that officers let Abdeslam go after checking his ID. They spoke on condition of anonymity, lacking authorization to publicly disclose such details.

It's not clear why the local French police didn't take Abdeslam into custody. They checked his identification, but it's not known whether they had been informed of his apparent connection to the attacks.

"It was a simple check. There was no lookout notice at the time of the traffic stop," a French police official told the AP.

Asked whether Abdeslam's name had been shared over police networks by then, the official simply said: "I have no explanation."

Warnings from Iraq

It may not have been the only missed opportunity before and after France's deadliest extremist attack since World War II.

The day before the attacks, senior Iraqi intelligence officials warned France and other members of the U.S.-led coalition fighting ISIS that assaults by the militant group could be imminent, according to a dispatch obtained by the AP. Abu Bakr al-Baghdadi, the group's leader, had ordered supporters to use guns and bombs and take hostages in the days ahead in coalition countries as well as Iran and Russia, Thursday's dispatch said.

The dispatch did not say where or when the attacks might take place, and a senior French security official told the AP that French intelligence gets this kind of communication "all the time" and "every day."

But Iraqi intelligence officials told the AP that they also shared specific details with French authorities before the attack — including the size of a sleeper cell of militants they said was directing attackers sent back to France from Islamic State's de-facto capital in Raqqa, Syria.

These additional details were not corroborated by French or Western security officials. But one U.S. official said Sunday that the evident weaponry skill displayed by the attackers suggests that they might have received training somewhere.

Nearly all the French, Iraqi and U.S. officials providing information for this story spoke on condition of anonymity because they lack authorization to share details publicly.

Arrests in Brussels

Abdeslam is one of three brothers believed to be involved; One who crossed with him into Belgium was later arrested, and another blew himself up inside the Bataclan theatre after taking the audience hostage and firing on them repeatedly.

Three Kalashnikovs were found inside another car known to have been used in the attacks that was found in Montreuil, an eastern Parisian suburb, another a French police official said.

As many as three of the seven suicide bombers were French citizens, as was at least one of the men arrested in the Molenbeek neighbourhood of Brussels, which authorities consider to be a focal point for extremists and fighters going to Syria from Belgium.

Belgian Interior Minister Jan Jambon, speaking to The Associated Press by phone, said suspects arrested in Molenbeek had been stopped previously in Cambrai, France, "in a regular roadside check" but that police had had no suspicion about them at the time and they were let go quickly.

One, identified by the print on a recovered finger, was 29-year-old Frenchman Ismael Mostefai, who had a record of petty crime and had been flagged in 2010 for ties to Islamic radicalism, the Paris prosecutor said. A judicial official and lawmaker Jean-Pierre Gorges confirmed his identity. A judicial official said police have also identified two other of the suicide bombers, both French nationals who'd been living in Belgium: 20-year-old Bilal Hadfi, who detonated himself outside the Stade de France; and 31-year-old Brahim Abdeslam, the brother of fugitive Salah Abdeslam, who blew himself up on the Boulevard Voltaire.

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

Panic at vigil

Outbursts of panic gripped several Paris neighbourhoods Sunday evening where vigils were being held in the wake of this weekend's suicide attacks, but they were false alarms, French law enforcement officials said. 

Paris Attacks - Notre Dame memorial

The mood was sombre as the bells of Notre Dame rang out in central Paris Sunday night ahead of a memorial service for the victims of the attacks. (Ellen Mauro/CBC)

A crowd at Place de la République apparently panicked after hearing firecrackers, one of the officials said. Someone reached out to police in the plaza out of fear, and when officers arrived with weapons drawn, the spooked crowd scurried, a security official said.

"It appears that someone thought it was a clever idea to set off firecrackers and see if they can scare people, and they panicked," said CBC's Keith Boag.

CBC's Thomas Daigle, reporting from the French capital, said calm was soon restored.

Still, despite the heightened sense of unease in the French capital, Boag says the situation isn't scaring residents into staying indoors.

"People I spoke to were first and foremost making the point they won't be frightened out of their neighbourhood," he said. "By and large, people want to be out in the streets tonight, and they want to show that they still own their city."

Vigils and church services were being held for victims of Friday night's attacks, which left 129 dead and more than 300 wounded following in a series of co-ordinated assaults on restaurants, a concert venue and a soccer stadium.

France Paris Attacks

A man carries two children after panic broke out among mourners paying their respects at the attack sites at restaurant Le Petit Cambodge and the Carillon Hotel in Paris, Sunday, Nov. 15, 2015. (Peter Dejong/AP)

Close to Place de la République, panic broke out near a small Cambodian restaurant and a bar that were the scenes of shooting on Friday night. Police were seen running with guns drawn, but a security official said there were no confirmed threats to the area.

The security officials weren't authorized to be publicly named, in accordance with government policy.

With files from CBC's Nahlah Ayed, Thomas Daigle and Keith Boag, and Reuters