The collapse happened at the site of a building under construction on Alpine Road.
Fire officials said the collapse caused three cranes to topple over, with one person inside each. Two of them were taken to a local hospital in unknown condition. The third person refused treatment at the scene.
Several cars were also damaged in the collapse.
While storms were moving through the area at the time of the incident, it was not yet known if weather caused the problems.
OSHA and the Winnebago County sheriff's department are investigating.