Around 30 people were also injured after a German citizen drove a van into a group of people in Muenster city, officials say, refusing to confirm local media reports that the perpetrator had psychological issues.
The parliament in Germany has agreed on measures to increase surveillance in public places to boost domestic security. But some are concerned over the restriction of liberties.
Still searching for clues about the Berlin Christmas market attack, police in Germany are conducting raids across the city.
The detained suspect reportedly had dinner with the the main suspect - Anis Amri - the day before the attack in Berlin that killed 12 people.
The man allegedly responsible for the Berlin Christmas market attack came from a small impoverished Tunisian town. His family and neighbours say the young man who left for Europe as a teenage delinquent was changed by his time in an Italian prison.
Germany was offering up to €100,000 ($104,000) for information leading to the arrest of the Tunisian suspect.
Germany is offering a €100,000 reward for information leading to the arrest of Tunisian man, Anis Amri.
German Federal Prosecutor's Office announces a reward of 100,000 euros for information leading to the arrest of suspect.
Berlin police are investigating whether Monday's attack at a Christmas market in the German capital was terror-related.
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