(Reuters) – Police raided homes of suspected Islamist militants across France overnight following Friday’s Paris attacks, and a source close to the investigation said a Belgian national in Syria was the possible mastermind.

Prosecutors said one of the killers had been stopped and fingerprinted in Greece last month, fuelling speculation that Islamic State had taken advantage of the recent influx of refugees fleeing the Middle East to slip militants into Europe.

The carnage, which killed 129 people, has led to calls for the European Union to close its borders to asylum seekers.

Islamic State, which has claimed responsibility for the attacks in retaliation for French airstrikes in Iraq and Syria, warned in a video on Monday that any country hitting it would suffer the same fate, promising specifically to target Washington.

French warplanes bombed Islamic State training camps and a suspected arms depot in its Syrian stronghold Raqqa late on Sunday — its biggest such strike since it started assaults as part of a U.S.-led mission launched in 2014.

Much of France came to a standstill at midday for a minute’s silence to remember the dead. Metro trains in Paris stopped, pedestrians paused and office workers stood at their desks.

Interior Minister Bernard Cazeneuve told reporters police had arrested 23 people and seized arms, including a rocket launcher and automatic weapons, in 168 raids overnight. Another 104 people were put under house arrest, he said.

“Let this be clear to everyone, this is just the beginning, these actions are going to continue,” Cazeneuve said.

The investigation into the coordinated Paris attacks, the worst atrocity in France since World War Two, led swiftly to Belgium after police discovered that two of the cars used by the militants had been rented in the Brussels region.

Dozens of Belgian police and armed commandos surrounded houses on Monday in the Brussels district of Molenbeek, which is home to many Muslim immigrants.

Police arrested seven suspects in Brussels over the weekend, five of whom have been released, and are hunting Salah Abdeslam, a 26-year-old Frenchman based in the Belgian capital, who is one of three brothers believed to have been involved in the plot.

A source close to the investigation said Belgian national Abdelhamid Abaaoud, currently in Syria, was suspected of having ordered the Paris operation. “He appears to be the brains behind several planned attacks in Europe,” the source told Reuters.

French prosecutors say they have identified five of the seven suicide attackers who died on Friday. Four were French, while the fifth man was stopped and fingerprinted in Greece in October and was possibly Syrian.

“We know that more attacks are being prepared, not just against France but also against other European countries,” Prime Minister Manuel Valls told RTL radio. “We are going to live with this terrorist threat for a long time.”

(Read more from Reuters here)