Four men were “seriously wounded” in a stabbing at a gym in western Germany Tuesday night, police said. Three of them suffered “life-threatening injuries.”

The incident happened in Duisburg, a city of half a million people about 50 miles north of Cologne, just before 6 p.m.

The victims, all German Citizens, are between the ages of 21 and 32, the German state-owned broadcaster Deutsche Welle reported, citing local police.

“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,” police said Wednesday morning.

Witnesses described the attacker as a male, about 30 years old, of normal build with a long black beard. He was wearing a long-sleeved pullover, dark trousers and a black baseball cap.

Police say he was likely acting alone.

No arrests have been made as of Wednesday morning. Investigators are still looking into a possible motive.

Herbert Reul, the interior minister for the state of North Rhine-Westphalia, told the dpa German Press Agency that investigators are treating the case as if it weren’t an indiscriminate attack, but he didn’t elaborate.

“I hope that the victims will recover quickly and that everyone who was on the scene and saw this terrible act will recover,” Reul said

Last month, six people were killed in the northern German city of Hamburg after a gunman stormed his former Jehovah’s Witness center, the Kingdom Hall house of worship in the Gross Borstel district, and opened fire.

Eight other people were wounded in the attack. The suspect took his own life after police arrived.

With News Wire Services