Special police forces arrested the 32-year-old man at about 8pm on Thursday, according to a police report, but they said that there was no evidence of concrete danger.
Police said that the German man has been long known to authorities. Regional broadcaster WDR reported that he had converted to Islam and was a part of the Salafist scene. He reportedly became radicalized after his conversion.
The suspect had also previously attracted the attention of police for other crimes, WDR reported.