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 a crime scene spanning blocks, strewn with abandoned chairs and flags. Witnesses who later came back to retrieve strollers and other items were told they could not go beyond police tape.

"It sounded like fireworks going off," said retired doctor Richard Kaufman, who was standing across the street from where the gunman opened fire, adding that he heard about 200 shots.

"It was pandemonium," he said. "People were covered in blood tripping over each other.”

The shooting comes with gun violence fresh on the minds of many Americans. Just hours after the shooting in Highland Park, two Philadelphia police offers were shot near the Benjamin Franklin Parkway as thousands of people celebrated a Fourth of July concert and fireworks show. Both officers were later released from hospital. read more

In May, a gunman murdered 19 school children and two teachers at an elementary school in Uvalde, Texas, just 10 days after a man shot dead 10 people at a grocery store in Buffalo, New York. read more

The Chicago suburb attack is likely to rekindle the debate about gun control and whether stricter measures can prevent the mass shootings that happen so frequently in the United States.

Police said they did not know what the motive was for the shooting in Highland Park. The wounded ranged in age from 8 to 85, including four or five children.