Police investigate an apparent bomb explosion at the Social Security Administration office on Marshall Street in downtown Casa Grande on Friday. No one was hurt in the explosion and federal authorities are conducting the investigation.

CASA GRANDE  — Authorities say they’re investigating a small explosion that happened at the back entrance of Casa Grande’s Social Security Administration office.

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

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

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives has investigators at the scene. ATF spokesman Tom Mangan says they are just starting their work and more information will be available later Friday morning.

Casa Grande police did not immediately respond to requests for information.

Senior Editor, News, Arizona Daily Star, Tucson, Az.