Four men were arrested on suspicion of obtaining guns before events marking the anniversary of German reunification, but officials said Sunday they found no weapons or evidence of plans for an attack.
Three of the men were arrested Saturday in the Bonn area, and the fourth in Offenbach, near Frankfurt.
Cologne police spokesman Carsten Moellers said that the men, aged between 22 and 27, were arrested because "we couldn't rule out that a crime was to be prepared"; and in view of the fact that celebrations marking the 21st anniversary of reunification are being held in Bonn on Monday.
However, police said that they found no firearms or other dangerous objects during searches of the men's apartments and that they ruled out the men, whom they described as belonging to the "Islamist spectrum," posed any danger to the events in Bonn.
The man arrested in Offenbach was released after a few hours. Cologne police said the other three would be freed late Sunday because of a lack of evidence to justify keeping them in custody, but they would remain under observation.
Germany's federal prosecutor's office also said there was no sign that the men belonged to any terrorist group and no indication of concrete plans for any attack.
The agency, which is responsible for terrorism cases, said it did not see grounds for it to take over the investigation.