Witnesses said the shooting took place in the 1000-block of East Jackson Street in the backyard of a home.
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"It was supposed to be a small group of people, a group of friends, but word got out and things turned left real fast," said Bobby Moore, a neighbor who attended the party.
Moore said the gathering spread on social media. As the small party quickly ballooned into 200 people just after 12:30am on Sunday, several gunshots rang out.
"I turned to my group of friends and said get on the floor, we were too close to the shooters," Moore recalled. "I looked back and seen the poor girl laying on the floor."
The girl he saw was Holly Mathews. The 22-year-old, who had just gotten engaged two weeks ago, was killed.
"She was just a young girl in a costume for a Halloween party," said her father, Stephen Mathews. "Lost her life for absolutely no reason, absolutely no reason."
More law enforcement officials arrived and saw multiple people who had been shot in the backyard and at nearby houses. More shots were heard while first responders were treating those who had been wounded, police said.
Jonathon Ceballos, 22, was also killed and 12 others were shot when two men standing on the porch fired into the crowd. Some neighbors believe it all could have been avoided if police responded to their calls earlier in the night.
"If you get several calls, why wouldn't you go and check and make sure everything is ok," neighbor Natalia Herrera said.
Herrera said her family called twice, as did many other neighbors who saw the party getting out of control.
"People just kept coming and coming, there were too many people standing on the street," Herrera said.
Moore's parents also called the police, but neighbors said the party kept going until the shooting.
"I think definitely with a police presence, the shooters would have gotten spooked and left the party," Moore said. "It couldn't have been prevented, but can't go back in time."
Because the house is in unincorporated Joliet, the Will County Sheriff's office is investigating. No one is in custody.
And through a press release, the sheriff's department defended its response to neighbor complaints about the big party. While not uncommon to receive numerous noise complaints on weekends and holidays, the department said a sergeant was in the vicinity after the calls and because of it, responded to the shooting quickly
Anyone with information, cellphone pictures or video of the party or potential suspect information is urged to contact the Will County Sheriff's Office at 815-727-8574, extension 4930, or via email at email@example.com.
Anonymous tips can be submitted on the Will County Sheriff's website at www.willcosheriff.org/enforcement/submit-a-crime-tip or on the Will County Sheriff's Office App that is available in iOS and Android.
You can also contact Will County Crime Stoppers by telephone at 800-323-6734 or at www.crimestoppersofwillcounty.org/submit-a-tip/.