“As far as we know, there are 20 people unaccounted for,” said Captain Stefanie Smith.
“We should take into consideration that those people could have possibly gone home or even been taken to hospital, so we are just trying to account for everyone who was supposed to be at work when the blast happened.”
The explosion sparked a fire which resulted in the evacuation of the building.
Thirty-five people sustained burn wounds in the incident and were taken to the Wilhelm Stahl Hospital for treatment.
The blast also damaged a part of the building and rescue workers were searching the area for people who might be trapped.
Police were investigating a case of arson.
The estimated damage had not yet been determined.