The blast was caused by a device that detonated at the rear of the building, according to Agustin Avalos of the city of Casa Grande Police Department.
The explosion happened around 8:15 a.m. in a building that houses several other downtown businesses. All business were evacuated, Avalos said.
No injuries were reported and the type of device had not been determined, Avalos said.
Fire Marshal Barbara Rice said a fire was quickly extinguished, but there was some damage to the exterior of the building.
A number of staff members were in the office at the time of the explosion, but no injuries were reported.
The building has been evacuated and the scene is cordoned off. Rice said nearby buildings were not evacuated.
Rice said the incident appears to be a deliberate act, but that has not been confirmed. Officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) are headed to the scene to investigate.
There is no suspect information at this time.