FBI probes scene of July 4 parade shooting in Chicago

Onlookers at the scene of the shooting (AP)

FBI agents are searching for more evidence at the site where a gunman killed at least six people after opening fired on a US Independence Day parade from a suburban Chicago rooftop.

The shots were initially mistaken for fireworks before hundreds of panicked revellers fled in terror in Highland Park, a close-knit community on the shores of Lake Michigan.