Authorities arrested a man during a traffic stop Friday in connection with an explosion at the Social Security office in Casa Grande, Arizona, a law enforcement official said.
The man is held under federal authority and faces possible state and federal charges, said the official with knowledge of the investigation.
"He was heading eastbound on the main road out of the Casa Grande area," the official told CNN.
The explosion occurred Friday morning outside a door, causing a small fire and minimal damage, said Casa Grande Fire Marshal Barbara Rice.
No injuries were reported, FBI spokeswoman Jennifer Giannola said.
The building was evacuated, but the surrounding buildings were not, Rice said.
FBI bomb technicians, the Bureau of Alcohol, Tobacco, Firearms and Explosives and hazardous materials experts were participating in the investigation.