The motive for the incident in Stade, close to the port city of Hamburg, was not immediately clear, according to a police spokesperson.
The role of the second individual in custody is currently unclear, a second police spokesperson told Reuters, adding that no other suspects were at large.
It is unclear how many people have been injured, he said, adding that the dead were all adults.
Local media reported that the incident had taken place near a youth centre in central Stade, a town of just below 50,000 people to the west of Hamburg. The spokesperson denied that, but declined to give further details.
Just after the incident took place, police had warned residents to steer clear of the area.
MNA
