
Fire Chief Insp. Lucio Albarracin attributed the blaze to a lighted cigarette butt thrown into a pile of papers inside a carton box.
Personnel from the City Security Unit stationed at City Hall used up to five fire extinguishers to stop the spread of the fire.
The fire was already under control when the firemen arrived in the area.
City Hall services were temporarily shut down after employees were ordered to get out of the building. Work is expected to resume this afternoon.
The Bureau of Fire Protection and the Office of the City Mayor are expected to conduct an investigation into the incident.
Albarracin is not discounting the possibility of arson, saying fire investigators have taken down the names of possible witnesses and are looking at all possible angles in their investigation.