(Rewrites, adds details)
PHOENIX, Nov 30 (Reuters) - A firebomb exploded on Friday outside a federal office building in southern Arizona, causing minor damage and prompting an evacuation of the facility, but no injuries were reported, authorities said.
Police arrested a man suspected of setting off the blast, which occurred at about 8:30 a.m. local time (1530 GMT) behind the Social Security Administration building in downtown Casa Grande, a small city about 50 miles (80 km) south of Phoenix, a source close to the investigation told Reuters.
The suspect, who faces an array of state and federal charges, was detained after he was pulled over while driving, said the source.
"They caught up to a lone individual on a traffic stop in a car, and he was subsequently taken into custody," said the source, who spoke on condition of anonymity because he was not authorized to discuss details of the case publicly.
The firebomb went off near a rear doorway to the building, and flames from the blast were quickly extinguished, Casa Grande Fire Marshal Barbara Rice said.
She said an unknown number of workers were ordered out of their offices as a precaution, and the building was cordoned off for investigation. No one was hurt, Rice said.
The flammable substance used in the bomb had yet to be determined but could have been gasoline, according to the source. It was not immediately clear how the device was detonated.
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, Firearms 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 Tim Gaynor and David Schwartz; Editing by Steve Gorman, Eric Walsh and Lisa Shumaker)