The gunman who killed six people Monday during the Fourth of July parade near Chicago planned his attack "for weeks" and disguised himself as a woman so as not to be identified, police said Tuesday.

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Robert Crimo, 21, used a 'powerful rifle' that appears to have been purchased legally at the parade and presumably fired randomly at attendees from a rooftop of a business, a local police official said during of a press conference. He had "dressed as a woman" to hide his identity and flee the scene more easily, he added.