By Brendan O'Brien and Kanishka Singh

HIGHLAND PARK, Ill. - The Chicago suburb of Highland Park had planned for a Fourth of July celebration like so many in communities across the United States: a parade followed by fireworks and music.

But the festivities to mark U.S. Independence Day turned into bloody mayhem as a gunman on a rooftop opened fire on families at the parade on Monday, killing six and wounding more than 36.

In response to the Highland Park shooting, numerous Chicago suburbs canceled their celebrations.

Sara Hainsfurther, a 36-year-old native of Highland Park who was at the parade with her family, said she had attended almost every year since she was a child.

"Not even five minutes after, very shortly after, the police and firetrucks part of the parade had gone by I heard 'pop, pop, pop, pop, pop'," Hainsfurther said.

"My mom said 'wow, those are really loud,' and I looked to see if they were muskets, because you know sometimes they'll use those old guns in the fourth of July parade. ... The popping didn't stop though, again it went 'pop, pop, pop, pop, pop' and I turned and I said 'those are gun shots, run'."

Another man at the parade said the shots initially sounded like fireworks but it soon became "pandemonium."

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

"It was pandemonium. A stampede. Babies were flying in the air. People were diving for cover," he said as he watched police work the scene. "People were covered in blood tripping over each other."

The main street in Highland Park became a crime scene spanning blocks, strewn with abandoned chairs and flags. Witnesses came back hoping to retrieve strollers and other items but were told they could not go beyond police tape.

As the shots rang out, some people took time to process what was happening before fleeing in panic.

Alberto Martinez, 33, who works at a nearby hospital, was at the event with his family.

"I froze for a moment. It was scary. It was a scary moment," he said, adding his wife picked up their three-year-old and they began running.

His family went back to his parents' house, locked the doors and watched the news.

"I cannot process it all. All the shootings need to stop," he said.