The attack was captured on cellphone video and posted on social media. 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.
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 record the beating.
On Thursday, Chicago police spokesman Anthony Guglielmi said three girls, ages 13, 14, and 15, have been arrested in connection with the attack and that charges include aggravated battery and mob action.
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.
She was taken to the 11th district where they decided to take her to the University of Chicago Medical Center.
Supporters of the teen said the incident began when the girl, who is a high school sophomore who suffers from a mental disability, was not only beaten but, they claim, sexually assaulted after she refused to perform a sex act on a man the group all knew in exchange for a free tattoo. Other teen girls allegedly did perform the act.