Police have three people of interest in custody and are looking for three others.
The victim was found unconscious around 9:30 p.m. in the 2100 block of North Rockwell Street, police said. She is being treated at Illinois Masonic Hospital.
Elijah Billot pointed to a cold patch of grass where he and relatives discovered the teenage victim.
"She was completely unconscious," Billot said. "She was bleeding from the back of her head. She had her pants down. And we, of course, the first thing we thought was that she was raped."
Billot's relatives covered the woman with a blanket until an ambulance arrived. Twenty-four hours later, they remain stunned by the crime.
"I have like I said two daughters and a granddaughter, and I cannot imagine the pain this mother is going through," witness Marcia Flores said. "It's horrible. I wouldn't know what to do."
It appears trouble began after witnesses said the woman could not get into a New Year's Eve concert by Rusko at the Congress Theater because she did not have an ID.
The performance by the prominent British DJ was open to patrons 17 or older but identification was required.
A theater worker said when she couldn't get in, she sat down at a nearby curb.
When the woman was approached by six men who had been at a nearby Mexican restaurant, they all walked down Rockwell together, according to the theater worker.
About 20 minutes later, neighborhood residents made the shocking discovery.
Concert security officers managed to detain three people who reportedly had blood on their clothes.
The woman, who remained unidentified until Sunday afternoon because she had no ID, remains in a coma at an area hospital.
Police hope surveillance video from a nearby restaurant will lead to arrests as a neighborhood remains on edge.