At least four people were seriously injured in a stabbing attack in a fitness studio in the centre of the western German city of Duisburg, police said on Tuesday evening.
"At least" one perpetrator had attacked several people at the gym located across from the Duisburg city hall, using a "stabbing or cutting weapon," according to police.
The people who were injured were being treated in hospital, the police said.
Police in Duisburg were alerted by emergency calls at around 5:40 pm (1540 GMT), they said.
Investigators searched the gym and secured it, police said shortly after 8 pm. Witnesses who were working out at the time of the attack are currently being questioned.
According to a dpa reporter at the scene, heavily armed special police forces were on site in the evening. Eyewitnesses were questioned by the police in a nearby restaurant, he reported, adding that one person was still carrying his sports bag.
A police spokesman said earlier there is no information yet on whether one or more perpetrators are still on the run.
Police called on citizens to avoid the area around the city hall, adding that a large police contingent was present.
Public broadcaster WDR reported earlier that two perpetrators stabbed visitors in a fitness centre in the old town of Duisburg, some 70 kilometres north of Cologne.
Germany's Bild newspaper reported that police suspect the perpetrator or perpetrators went on a rampage and are still on the run.
