Authorities are investigating a small explosion that happened at the back entrance of a Social Security Administration office in a small town about 50 miles south of Phoenix.

The explosion was reported just before 8:30 a.m. Friday and no one was injured, Casa Grande Fire Marshal Barbara Rice said.

Television images show the building was blackened around the entry door, but Rice said the damage was not extensive.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives sent investigators to the scene. ATF spokesman Tom Mangan said the FBI had taken over the investigation and referred questions to that agency.

Calls to the FBI and Casa Grande police were not immediately returned.

A local resident whose wife works in the office and who talked to one of her co-workers said he was concerned about the staff, even though no one was hurt.

"She was crying and extremely upset," Ed Mouradian told the Arizona Republic. "I am concerned for all the employees' well-beings. It's a small office that's very tight-knit."

Casa Grande was once a farming community of a few thousand people but grew into a distant bedroom community for Phoenix when developers began building hundreds of homes in the early 2000s.