(JTA) — It was the threat that European authorities dreaded — and Europe’s Jews suffered the first blow.

The suspect arrested in the attack last month at the Jewish museum in Brussels that left four dead was a French-born jihadist who had returned home from fighting in Syria.

Now European Jewish institutions are left to reckon with the danger of European jihadists coming home from Syria with deadly new skills, extremist fervor and malicious intentions.

“There has been a change and it requires us to fundamentally reconsider the degree of threat posed to Jewish targets not only in France, but across Europe,” said Sammy Ghozlan, a French former police officer and president of the National Bureau for Vigilance Against anti-Semitism. “That is the only way to prevent attacks like the one in Brussels.”

On Friday, police in Marseille arrested Mehdi Nemmouche, 29, on suspicion that he carried out the May 24 attack at the Jewish Museum of Belgium. French police found an assault rifle, handgun and a small video camera in Nemmouche’s bag.

Nemmouche, who was born on France’s border with Belgium, is believed to have traveled via Brussels in 2012 to fight with jihadists in Syria’s civil war. Western intelligence agencies have feared that European Muslims fighting in Syria will return and commit terrorist attacks in their home countries.

French Prime Minister Manuel Valls said in January that the threat of jihadists returning to Europe is “the greatest danger that we must face in the coming years.” He added, “It’s a phenomenon of unprecedented size.”

Ghozlan has called on his government to revoke the citizenship not only of jihadists who leave to fight but also of their families.

“Our synagogues and schools already resemble fortresses,” he said. “It’s time for the perpetrators, not the victims, to fear for their families.”

France already has hardened its line on French nationals who undergo Islamist indoctrination and weapons training abroad as part of its security services response to the actions of Mohammed Merah, a 23-year-old Muslim radical from Toulouse who in 2012 killed three soldiers and four Jews.

Merah, who died in a shootout with police, had undergone training in Pakistan and Afghanistan and visited Syria and Jordan two years before the murders. He had surveyed and filmed Toulouse’s Ohr Hatorah Jewish school many days before he killed three children and a rabbi there.

To the Israeli Jewish Congress, a 2-year-old group that aims to strengthen ties between Israeli and European Jews, the phenomenon means that perpetrators of anti-Semitic attacks ”are becoming much more sophisticated and professional in their combat training.”

The danger is thus “exacerbated not only from professional lone wolf attacks like in Brussels, but potentially also attacks on a much larger scale,” said Arsen Ostrovsky, the group’s director of research.

Experts on the security of Jewish institutions in five countries told JTA that since the war in Syria, they have observed a substantial increase in cases involving the gathering of intelligence on Jewish institutions by unidentified individuals.

“We see the gathering of tactical intelligence on Jewish targets occurring more often, we have security camera footage of it happening,” said Michael Gelvan, the Copenhagen-based chairman of the Nordic Jewish Security Council, which serves the Jewish communities in Denmark, Finland, Norway and Sweden. “It suggests the emergence of new and very serious threat which, unfortunately, not everyone has understood.”

Two days after the Brussels museum attack, a police receptionist near Paris received a report that lit up all sorts of warning lights at her emergency call center: Three men had been seen pointing a small video camera at the entrance to the local Otzar Hatorah Jewish school, according to the report given by the receptionist to the dispatch unit on May 26.

Officers hurried to the scene, but the three men had fled, realizing they had been spotted, according to a report by the Le Parisien daily. Some of Europe’s Jewish communities spend more than a quarter of their budgets on security, according to the European Jewish Congress.

Sophisticated attacks that entail surveillance, planning and prowess are nothing new for European Jews, who have seen many attacks by Palestinian terrorists during the 1970s and 1980s, noted Ghozlan, the ex-French police officer.

Some of the deadliest attacks occurred in Antwerp, where terrorists in 1981 detonated a car bomb near a synagogue, killing four people.

Yet the terrorist groups were limited in the number of attacks they could carry out because their operations required substantial investment in training operatives and covertly sending them abroad, whereas “thousands of European Islamists operating independently constitute a drastic quantitative change from a risk-assessment point of view,” said a spokesman from British Jewry’s Community Security Trust, or CST.

Ghozlan also identifies a qualitative change.

“The fervor introduced by Islamist indoctrination creates a new kind of determination in people who believe that killing a Jew is their ticket to heaven,” he said. “In a sense, we are dealing with [the equivalent of] kamikaze.”

Cnaan Liphshiz is JTA’s news and features correspondent in Europe. Based in the Netherlands, he covers the mosaic of cultures, languages and traditions that is European Jewry. Born in Israel, he used to work as foreign news editor for Ma’ariv and as a reporter for Haaretz.