Police in Cologne said Wednesday they were investigating an explosion that tore through a cafe in the early hours of the morning.

The blast took place shortly before 3 a.m. local time (0100 UTC) in the northwestern district of Pesch, according to police.

The cafe, which occupies the ground floor of an apartment building, reportedly went up in flames.

Around 20 people were evacuated from the building, while two residents were treated for suspected smoke inhalation, a spokesperson said.

Suspect surrenders to police

Hours after the incident, police said a suspect accompanied by a lawyer had handed himself in. They later said the man had been released after police searched his home and questioned him over the blast.

A search is currently underway for a second suspect.

Cologne Mayor Henriette Reker said she hoped the investigation would be concluded swiftly. 

Authorities said initial findings from their investigation suggested the blast was not linked to a series of explosions that have hit different locations in Cologne in recent weeks.

Last week, there were two blasts in the city center in the space of three days.

Police say they believe the explosions may be linked to rival gangs involved in the illegal drug trade.

"There are obviously unpaid bills in the milieu that are still being settled," Michael Esser, the head of Cologne's criminal investigation department, said last week, calling the violence "unprecedented."

nm/dj (AFP, dpa)