Robert Crimo, the 21-year-old arrested for the July 4 parade massacre in the Chicago suburb of Highland Park which left seven people dead and 39 wounded, planned the attack for weeks and dressed as a woman while carrying it out so as not to arouse suspicion before the incident and while he made his getaway, authorities said Tuesday.

Lake County Sheriff's Department spokesman and Deputy Chief Christopher Covelli provided this information at a press conference after the department interviewed witnesses to, and survivors of, the attack and reviewed cellphone video recordings made during the shooting spree.

Shortly after the press conference began, Highland Park Mayor Nancy Rotering confirmed to CNN that a seventh person had died.

So far, police have not been able to establish Crimo's motives for the attack, and charges have not yet been filed against him.