Police in Duisburg believe a man aged around 30 acted alone when stabbing and seriously wounding four people at a gym in the western German city on Tuesday evening. 

The suspect was still on the loose on Wednesday morning following an unsuccessful manhunt after the suspected knife crime.

Investigators have opened a preliminary attempted murder case. 

There are no surveillance cameras in the gym in Duisburg's old town. Police, therefore, have been struggling to reconstruct the exact course of events. Investigators do not suspect a homicide but a confrontation of one or more of the victims with the perpetrator, the public prosecutor said.

"There are no indications of a rampage," she said. The exact circumstances of the altercation are still under investigation, she added.

What condition are the victims in?

Four people aged between 21 and 32, all German citizens, were injured in the stabbing.

"Three men suffered initially life-threatening injuries and one man was seriously wounded," police said. "They were taken to nearby hospitals and in some cases underwent emergency procedures. According to our current information, one man's life is still in danger."

The first emergency calls from the fitness studio came in just before 6 p.m. on Tuesday evening. 

Police gave no information on a possible motive or background to the attack. 

But the interior minister for the state of North Rhine-Westpahlia, Herbert Reul, told German news agency dpa that police were currently working on the assumption that the attack had not been indiscriminate. 

"I hope now that the victims will heal quickly and that everyone who was at the scene and saw this terrible crime will recover," Reul said. 

What do police know about the suspect? 

Police said they could only offer a preliminary description of the suspect because the strongest witnesses had required medical attention before they could be debriefed.

"Several witnesses were questioned in the evening and into the night," police said in a press release. "Because the four injured people were not immediately able to be interviewed, a precise description of the suspect could not at first be verified."

A full description would follow as soon as possible, they said.

But police said the individual was believed to be aged around 30, roughly 1 meter 80 centimeters (just under 6 foot) tall, of a normal build with a long black beard. They described his appearance as reminiscent of someone from a "southern country," a common German police euphemism to denote a dark-skinned suspect.

He was thought to have been wearing a dark, long-sleeved pullover and dark trousers during the attack, as well as a black cap with its bill facing the front.

Gym sealed off for evidence-gathering

Police sealed off and searched the gym seeking further evidence and in a bid to limit any potential further threat. 

"The police's search operations are running at full speed," police said earlier on Wednesday, adding that they had set up an online portal where they were appealing to possible eyewitnesses for videos, photos, or written evidence pertaining to the attack.

Anonymous tips were also welcome, they said.

msh/nm (AFP, dpa)