Police say an explosion and fire started at a Social Security Administration building near Florence and 2nd Streets on Friday.
Officers say someone threw something out of a car window at the offices, causing a fire to start just before 8:30 a.m.
There was minimal damage to the building -- the entry door was blackened and charred -- but no one was hurt.
However, people working in the surrounding businesses felt their buildings shake during the explosion.
The suspect is described as a Hispanic man driving a black Hyundai.
Agents with the FBI and the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives are investigating the explosion.
"We are working with our federal, state and local partners on this investigation. At this time we are utilizing all necessary resources to include: investigative personnel, Evidence Response Team, Special Agent Bomb Technicians and our Hazardous Materials Response Team... we are unable to confirm or deny whether anyone is in custody," stated a spokesperson for the FBI Phoenix Division.
The motive for the attack is unknown.