The victim's family said the teen is currently in the care of the Illinois Department of Child and Family Services as the investigation continues.
Four girls have been arrested in connection with the attack that was captured on cell phone video and posted on social media, police said Thursday. One of the girls is 13, another is 14, and two of the girls are 15, police said. They face charges that include aggravated battery and mob action.
Details of the girls' identities have not been released because they are juveniles.
The video shows the victim surrounded by at least five other teen girls and a teen boy before she's punched repeatedly as she screams for help.
"I haven't been able to sleep since I saw that brutal attack on my granddaughter," said Charlie Mae Harris, the victim's grandmother. "It was so terrible."
Friends of the 15-year-old victim said the group offered to walk with the teen to the CTA Red Line, but said it was all just a set up so they could jump her and videotape the beating.
Police said the attack happened some time during a five day span the victim had been missing from home. She was located by another friend and the friend's mother riding the Blue Line Tuesday, the same day her family reported her missing.
The victim's family hopes she will be relocated to live with a family member who is able to properly care for the teen.
"She's in an excellent school," said Raquel Harris, the victim's aunt. "She needs to continue with her life from this point on, but with family members that can truly protect her."
The victim is currently in the custody of the Illinois Department of Children and Family Services. They released a statement Thursday night, saying, "DCFS is investigating allegations of neglect involving the 15-year-old with special needs that was brutally attacked this week. The tragic incident is heartbreaking. The department is committed to protecting the state's most vulnerable children. We are working closely with law enforcement to fully investigate what happened in this case. DCFS opened the investigation following a hotline call on July 23 and took protective custody of the 15-year-old on July 24. The department has had prior contact with the family."