The FBI is investigating an explosion at a federal building in downtown Casa Grande on Friday. Sources tell 3TV that authorities are questioning a person of interest in the case. Police and fire crews responded to the explosion at the Social Security Administration building, 501 N. Marshall St., at approximately 8:15 a.m. The building is across the street from the Casa Grande Police Department. Upon arrival, police and fire officials learned that the explosion came from a device that detonated at the back door of the building, according to city spokesman Agustin Avalos. The device is unknown at this time.