“According to current information, a person has injured other people with an object” at a gym, Duisburg police tweeted. “The situation is currently still confusing.”
The attacker appeared to still be alone, the German news agency dpa reported.
Several victims were taken to a hospital, while investigators searched the gym, dpa reported. Several witnesses to the attack, some still with their sports bags, were taken to a nearby restaurant so police could question them.
Police said they received the first emergency calls around 5:40 p.m. local time.
Duisburg has about 500,000 inhabitants and is located at the confluence of the Rhine and Ruhr rivers. It was once famous for its coal mining and steel production, but in recent decades it has been plagued by high unemployment and poverty.
The speaker of the German parliament, Baerbel Bas, who is from Duisburg, was shocked by the attack.
“Terrible. My thoughts are with the injured,” Bas tweeted. “I hope the emergency services can clear up the threatening situation soon.”