BERLIN (AP) — Police are searching for a man suspected of carrying out a knife attack at a gym in the western German city of Duisburg that left four people seriously wounded, officials said Wednesday.

Three people were in life-threatening condition, and one person was less seriously wounded, Duisburg police tweeted late Tuesday. They said all four victims are German citizens but didn't immediately provide further details.

Duisburg prosecutors said there had been no reportable developments in the case overnight.

Authorities didn't rule out the possibility of more than one attacker being involved, saying that “at least one perpetrator is on the run.”

Police said that a “stabbing or cutting weapon” was used in the attack at a gym belonging to the John Reed Fitness chain.

The company said in a statement that it deeply regretted the incident and expressed hope that the victims would recover quickly.

Last month, a gunman stormed a service at his former Jehovah’s Witness hall in the northern Germany city of Hamburg, killing six people before taking his own life after police arrived.