BERLIN (Reuters) - Three Syrian men with suspected links to Islamic State have been arrested in Germany on suspicion of planning a large-scale attack in the western city of Duesseldorf, the federal prosecutor said on Thursday.

The plot was uncovered because a fourth Syrian man, identified as Saleh A., voluntarily turned himself in to authorities in Paris on Feb. 1 and confessed to the plot.

After the confession, it took German investigators four months to accumulate enough evidence against the three men to arrest them. Saleh A. remains in custody in France and Germany is seeking his extradition, the prosecutor said.

The arrests, made about two months after suicide bombers killed 32 people in Brussels, are likely to deepen concerns that Germany has become a priority target for Islamic militants operating in Europe.

Germany has not been the victim of a major attack like those seen in Belgium, France, Spain and Britain. But it is seen as particularly vulnerable because hundreds of thousands of migrants streamed into the country over the past year, overwhelming authorities who could not vet many of them.

German intelligence officials have said they believe IS militants took advantage of the chaotic influx to sneak fighters into Germany.

Prosecutors said in their statement that they believe Saleh A., 25, and one of the three men arrested in Germany, a 27-year-old named Hamza C., joined Islamic State in Syria in the spring of 2014 and received orders to carry out an attack in the old town of Duesseldorf.

IS leaders sent them to Turkey in May 2014, prosecutors said. The two men are believed to have then traveled separately through Greece to Germany in March and July of last year.

By January 2016, they had convinced a third man, Mahood B., 25, to participate in the attack, prosecutors said. Around the same time they contacted a fourth man, Abd Arahman A., 31, whose task was to make suicide vests.

Prosecutors believe Abd Arahman A. had worked previously for the militant group Nusra Front in Syria making suicide vests and bombs before he was sent by IS leaders to Germany in Oct. 2014 to participate in the planned attack.

SUICIDE BOMBINGS

According to the prosecutors’ statement, two of the men were planning to blow themselves up on the Heinrich-Heine-Allee, a busy road in the Duesseldorf city center, while other attackers would mow down pedestrians with guns and explosive devices.

There is no evidence the suspects had begun implementing their attack plans. The three men were arrested in the states of North Rhine-Westphalia, Brandenburg and Baden-Wuerttemberg. Their homes were also being searched.

“Germany, just like other European countries, finds itself in the crosshairs of international terrorism,” a spokeswoman for German Interior Minister Thomas de Maiziere said.

“We will do everything in our power to prevent terrorist attacks and protect people in Germany as best we can,” German Justice Minister Heiko Maas added.

The accusations are interesting because they suggest that Islamic State was deploying its members to launch attacks in Germany as early as 2014.

Authorities said earlier this month they were investigating 40 cases in which Islamic militants are suspected of having entered the country with the recent flood of refugees from the Middle East, which began last summer.

The number of migrants entering Germany reached peaks of more than 10,000 a day last autumn, but has fallen dramatically in recent months due to the closing of the Greek border with Macedonia and a deal between the European Union and Turkey that has discouraged refugees from crossing the Aegean Sea.

The reduction in the numbers has eased pressure on German Chancellor Angela Merkel, who came under fierce criticism last year for welcoming hundreds of thousands of migrants with the optimistic slogan “We can do this”.