By Eric Cox and Brendan O’Brien HIGHLAND PARK, Ill. (Reuters) -A gunman perched on a rooftop opened fire on families waving flags and children riding bikes at a Fourth of July parade on Monday, killing six and wounding more than 36 in the Chicago suburb of Highland Park. The gunman climbed to the roof of a business using a ladder in an alley, police said. The attack turned a civic display of patriotism into a scene of mayhem. Hours later, police announced that they had a suspect in custody after 22-year-old Robert E. Crimo III surrendered to authorities. The main street in Highland Park became…