The man who died was a 78-year-old tenant in whose flat the explosion is believed to have occurred.
According to police, emergency responders found several liquid gas bottles and heating devices that use gas in the flat, as well as World War ammunition.
According to Mayor Heiko Breithaupt, the explosion may have been caused by one of the gas bottles as the apartment building was not connected to gas for heating.
Police representative Frank Goetze told journalists, that there were no indications that explosives were in the house.
The Police had initially said that at least 25 people were injured, but later revised that number.
Breithaupt said one of the victims was seriously injured with burns, three emergency responders suffered from smoke inhalation.
The large explosion happened shortly before 9 am (0800 GMT) in the building on the edges of the central German town, which is located near the Harz Mountains.
The district fire chief, Kai-Uwe Lohse said the building was evacuated and experts were due to assess its structural integrity on Friday adding that its slabs had shifted.
Large parts of the surrounding area were cordoned off by police. Numerous police and fire brigade officials were on the scene, with helicopters circling over the health resort town.
The police said a nearby day-care centre was evacuated, with around 100 children unharmed, they were moved to other facilities.
A house across the street was left with cracked windows, while cars parked on the street also appeared to have sustained significant damage.
A 63-year-old resident of the area told dpa that a pen she was holding had fallen out of her hand.
“I was sitting in the kitchen and heard a big bang,’’ the resident said. (dpa/NAN)