A Garda armored car was making a delivery to a currency exchange in the 4300-block of West 26th Street at about 9:30 a.m., the FBI said.
After the driver exited the vehicle, a suspect wearing reflective vest struck him with a sledgehammer while trying to rob him, the FBI said. The Garda employee then shot the attempted robber.
The wounded suspect fled the scene and was observed by bystanders in a vehicle at a nearby intersection, the FBI said. The suspect was taken into custody by Chicago police and was hospitalized in critical condition, police said.
The Garda employee suffered non-life threatening injuries.