PHOENIX (Reuters) - Police have arrested a man after a firebomb exploded outside a federal building in southern Arizona on Friday, causing minor damage and prompting an evacuation of the complex, although no injuries were reported, authorities said.

The firebomb detonated at about 8:30 a.m. near a rear doorway to the Social Security Administration building in downtown Casa Grande, a small city about 50 miles south of Phoenix.

A male suspect was later arrested by Casa Grande police officers and is in custody facing an array of state and federal charges, a source close to the investigation said.

The accelerant used in the firebomb has yet to be identified, but could have been gasoline, the source, who declined to be named as he was not authorized to speak on the record, told Reuters.

Flames from the explosion were quickly extinguished, Casa Grande Fire Marshal Barbara Rice said, adding that an unknown number of workers were ordered out of their offices as a precaution, and the building was cordoned off for investigation.

Local television news footage of the aftermath showed the frame of the metal security door blackened by the blast and fire.

The FBI was leading a probe of the incident, since it occurred at a federal building, assisted by investigators from the U.S. Bureau of Alcohol, Tobacco, Fire and Explosives and local law enforcement.

When asked if he could confirm the arrest of a suspect, an FBI spokesman declined to comment.

(Reporting by David Schwartz; Editing by Steve Gorman and Eric Walsh)