The man accused of attacking a Fourth of July parade in a Chicago suburb bought his rifle legally, fired more than 70 rounds from a roof and dressed in women's clothing to blend into the fleeing crowd afterwards, local officials said on Tuesday.

The suspect, 21-year-old Robert E. Crimo III, surrendered to police on Monday, hours after the attack in Highland Park, Illinois, that took seven lives and sent over three dozen other people to hospital with gunshot wounds and other injuries.

Police revised the confirmed casualty toll on Tuesday with the death of a seventh person who had been hospitalized after the attack.

Among the dead were Nicholas Toledo, a grandfather from Mexico in his 70s celebrating with his family among the flag-waving crowds at Monday's parade, and Jacki Sundheim, a teacher at a nearby synagogue.

Officials told reporters the suspect had planned the attack for several weeks and fired into the crowd at random. Authorities were still considering what criminal charges to bring. It was not immediately clear if Crimo had a lawyer.

At a news briefing on Tuesday, authorities cited two previous encounters between Crimo and law enforcement - an April 2019 emergency-911 call reporting he had attempted suicide and another in September of that year regarding alleged threats he had directed a family members.