"A man, 53, was slightly injured in an explosion in a building entrance on the inner-city Hohenzollernring on Monday morning (September 16) at around 5.45 a.m. The explosion caused severe damage to the doors and windows of the building," the police and Public Prosecutor's office said in a statement.

Witnesses called the emergency services at around 5.45 a.m. after hearing a loud bang, it said.

Based on police video surveillance recordings, the police said that they are searching for a man who is believed to be connected to the crime and who ran away in the direction of Friesenplatz after the explosion.

The police described the suspect as wearing a white and blue jacket, dark trousers, and dark sneakers, and said any information about the crime and/or the man can be given over the phone or by email.

The NRW-Koln Police, in an X post in German, earlier in the day announced that it had launched a "major police operation in area #Hohenzollernring" and that the area was closed "between Rudolfplatz and Friesenplatz" and citizens should avoid the area.

Subsequently, it posted that the traffic closures on Hohenzollernring had been lifted, and further information on the police operation would follow shortly.

Local media outlet EXPRESS.de cited a police spokeswoman telling it that an explosion occurred between Rudolfplatz and Ehrenstrasse at around 5.50 a.m. "We have cordoned off a wide area and the investigation is ongoing," said the spokeswoman.

It said that the blast occurred directly in front of the Vanity Club Cologne.

It also claimed that it had obtained CCTV footage of a man in a hoody who apparently left a bag outside one of the entrances and the explosion occurred shortly after it.

It cited a local resident as saying that he was woken up early in the morning by a loud bang, saw a fire outside, and called the fire service.

He also claimed that he had seen several individuals running away from the area.

The Kolner Stadt-Anzeiger newspaper also reported that the incident may have taken place at a nightclub.

Radio Koln said the explosion also reportedly led to a fire, which had, however, been doused by the time firefighters arrived. It said that one person had suffered mild injuries and was taken to hospital.