Germany is investigating about 180 terror suspects who have returned from Syria or have links to militant groups there, the Justice Ministry said on Friday, a day after three Syrians were arrested on suspicion of planning attacks in Duesseldorf.
An Interior Ministry spokesman said it was too early to draw conclusions about the seriousness of the threat after the arrests over plans for a large-scale attack in the western city. But the security alert remained high, he said.
"At present the Chief Federal Prosecutor is conducting some 120 investigations into more than 180 suspects in connection with the civil war in Syria due to membership or support of a terrorist group," a Justice Ministry spokesman said.
German media have quoted security officials as saying the three arrested Syrians were registered at shelters for asylum seekers.
Der Spiegel, a German newsmagazine, said a fourth militant who was arrested in February had told authorities that the group had planned to carry out the attack in Duesseldorf with a total of 10 militant fighters.
The plan involved suicide attacks by two members, and bombing and rifle attacks, according to the magazine.
It said authorities had investigated the group for months, but decided to carry out the arrests after learning that one of the men had planned to travel to southern Europe. They were concerned that he could disappear or try to bring more recruits to Germany.
Johannes Dimroth, the Interior Ministry spokesman, told reporters on Friday that there were an estimated 500 potential Islamist extremists in Germany.
But Germany's police union chief Rainer Wendt cautioned against any temptation to cast suspicion on all migrants after the arrests.
"We know since the attacks of Paris and Brussels that the Islamic State wants to influence the migration debate in Europe and to whip up sentiment against refugees," Wendt told Reuters.
"This is part of their strategy. We must not fall into their trap," he said.
The influx of more than one million mainly Muslim migrants into Germany last year has raised concerns about both its ability to integrate them and possible security threats especially after attacks in France and Belgium by IS militants who had easily crossed open European borders.