A terrorism suspect is brought to the committing magistrate at the Federal Supreme Court in Karlsruhe, Germany, on Thursday. Three Syrians have been arrested over alleged plans to carry out a terrorist attack in Düsseldorf. (Uli Deck/European Pressphoto Agency)

Three Syrian men who entered Germany with a wave of migrants were arrested Thursday on suspicion of planning an Islamic State attack on the city of Düsseldorf. The arrests potentially thwarted a deadly operation that appeared eerily reminiscent of recent assaults on Brussels and Paris.

The suspected plot, German authorities said, involved suicide bombers, firearms and explosives — a lethal combination that has become the hallmark of a new spate of Islamist terrorism in Europe. A fourth Syrian, who prosecutors said had informed French officials about the alleged plot, was being held in France.

The arrests highlighted the significant threat to Europe from Islamic State militants posing as migrants. Officials said all four Syrians entered the continent from the Middle East using the same irregular passages by land and sea — Greece via Turkey and then through the Balkans — used by hundreds of thousands of asylum seekers last year. After the attacks in Brussels and Paris, Islamic State officials have claimed that more sleeper cells were incubating in Europe. Thursday’s arrests suggested such threats were not idle.

[Tracing the path of four terrorists sent to Europe by the Islamic State]

The German chief prosecutor’s office said in a statement that there were no immediate indications that the men had started taking concrete steps to carry out the plot. But the authorities moved in on Thursday — arresting the men in three German states — after details of the alleged plot were provided by the suspect in France, who first approached authorities in Paris in February.

People walk through Düsseldorf’s Old Town on Thursday. A German terrorist cell was allegedly planning an attack on the historic town center. (Maja Hitij/European Pressphoto Agency)

The plot, officials said, was supposed to involve two suicide bombers. Other assailants “were supposed to kill as many bystanders as possible with guns and other explosive devices,” prosecutors said.

Two of the men were suspected of being active members of the Islamic State, while a third was believed to have at least supported the group. Investigators also suspect that one of the two Islamic State adherents had links to the radical Islamist group Jabhat al-Nusra, which is known as the Syrian affiliate of al-Qaeda.

[Paris attacks were carried out by three groups linked to Islamic State]

Revelations that the suspects had entered Germany as migrants quickly fueled the debate here over the security threat presented by a massive pool of poorly screened asylum seekers. Hundreds of thousands of would-be refugees entered Germany last year after receiving only cursory vetting in near-bankrupt Greece. Over the past six months, more than three dozen suspected militants impersonating migrants have been arrested or died while planning or carrying out terrorism. They include at least seven directly tied to the attacks in Paris and Brussels.

Although a tenuous deal between the European Union and Turkey has largely blocked new migrants from entering Europe via Greece, more than a million have already arrived. Only a small fraction, officials say, present genuine security threats. But on Thursday, critics took aim at a haphazard migrant policy that was riddled with risk.

Gregor Golland, a Christian Democratic Union member of the North-Rhine Westphalia state parliament, called local leaders “naive” for insisting that “no terrorists were coming to Germany via the Balkan route” — a reference to the main land corridor used by irregular migrants last year.

[Brussels terrorist attack exposes security failures]

“Until today, we don’t know the identities of all refugees living in Germany,” Golland told the Rheinische Post. “The security authorities in North Rhine-Westphalia must organize an immediate security check of all refugees living in the country.”

Others called for calm, warning against fanning a growing strain of anti-refugee sentiment that has led to a surge in attacks by right-wing extremists on asylum centers and migrants.

“Of course one has to take this very seriously,” Düsseldorf Mayor Thomas Geisel told the local news website Report-D. “But the city must not lose its openness to the world — and tolerance.”

A series of coordinated terrorist attacks killed 130 people in Paris in November. The Islamic State claimed responsibility for the massacre, in which at least eight assailants armed with explosives and automatic weapons gunned down people at random at several locations in the French capital.

The Islamic State also claimed a series of attacks in Belgium in March targeting Brussels Airport and a Metro station. Three suicide bombings killed 32 people and injured more than 300 as authorities closed in on suspects wanted in the Paris attacks.

Germany’s chief federal prosecutor identified the three arrested Syrians as 27-year-old Hamza C., 25-year-old Mahood B. and 31-year-old Abd Arahman A.K. It is customary in Germany to withhold the last names of suspects who have been arrested.

[What we know about the Paris attacks]

The men were apprehended in the states of North Rhine-Westphalia, Brandenburg and Baden-Württemberg. Their apartments were being searched, officials said.

The Germans acted based on the information provided by a 25-year-old Syrian identified as Saleh A., who turned himself in to Paris police in February. His testimony alerted authorities to a German terrorist cell seeking to orchestrate an attack in the historic town center of Düsseldorf. The targeted area is known here as “the longest bar in the world” because of its concentration of beer halls and pubs.

Salah A. and Hamza C., authorities said, joined the Islamic State in the spring of 2014. Shortly after, in May, the organization’s leadership gave them fresh orders to carry out an attack in Germany. Two attackers would each detonate suicide vests on Düsseldorf’s busy central boulevard, Heinrich-Heine-Allee. Afterward, other attackers would kill bystanders using weapons and explosives.

With the approval of the Islamic State leadership, authorities said, Saleh A. and Hamza C. traveled to Turkey in May 2014. From there, they entered Europe separately. They first came in through Greece and then used the Balkan route, traversed by hundreds of thousands of migrants last year, before respectively arriving in Germany in March and July of 2015.

No later than January of this year, authorities believe, Saleh A. and Hamza C. persuaded Mahood B. to take part in the attack. The men were later joined by Abd Arahman A.K., another Syrian who had already traveled to Germany in October 2014, allegedly also on orders of the Islamic State’s leadership to take part in the attack.

Abd Arahman A.K. had experience building explosive belts and making grenades in Syria in 2013. Authorities believe that he was drafted to build suicide belts for the members of the German cell.

Authorities would not say what prompted Saleh A. to reveal himself to French police. He is still in custody in France, but German authorities are requesting his extradition.

Thursday’s arrests, officials said, had no connection with the upcoming European soccer championship in France. Fears have swirled in European security circles that the month-long event may be targeted by extremists.

Stephanie Kirchner in Berlin and William Branigin in Washington contributed to this report.

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