BERLIN — German authorities arrested three Iraqi refugees on Wednesday on allegations they were planning an Islamic extremist bombing attack, and searched properties in three states in connection with their investigation.

Federal prosecutors said that Shahin F. and Hersh F., both 23, and Rauf S., 36, were taken into custody in an early morning raid by a police SWAT team in the area of Dithmarschen, near the border with Denmark.

The suspects, who had refugee status in Germany, had been under surveillance for some time by a task force of around 200 investigators, said Holger Muench, the head of Germany’s federal police.

‘‘The case shows that the threat of Islamic terrorism is still present,’’ Muench said.

More than 1 million asylum-seekers entered Germany in 2015-16, most from Syria, Iraq, and Afghanistan.

The involvement of several asylum-seekers in extremist attacks or plots has helped boost support for the antimigrant Alternative for Germany party.

The two younger men are suspected of preparing a bomb attack and violating weapons laws, and the older one is alleged to have aided them.

The men appear to have been in the early stages of planning.

associated press