An explosion at a residential building in Germany that left nine first responders seriously injured appears to have been a planned attack carried out by a man wanted in connection with an unpaid fine, officials said Friday.

Police and firefighters had gone to the building in the western town of Ratingen on Thursday after receiving a call about a helpless person inside a top-floor apartment.

What started out as a routine operation turned into an inferno when a man opened the apartment door and threw out a burning object that exploded into a fireball, severely injuring the first responders.

“We are deeply shocked,” said Thomas Hendele, who heads the administration in Mettmann county, of which Ratingen is a part. “We are angry. Angry at this act, for which there is no justification.”