Chicago police on Wednesday were looking for up to six suspects who allegedly raped a teenage girl and broadcast the assault on Facebook Live.
The incident took place after the 15-year-old went missing Sunday. She was found two days later.
Chicago's police chief became personally involved in the case after the girl's mother approached him on Monday and showed graphic photos taken from a Facebook Live video, showing several young men assaulting the teenager, according to US media.
"It's disgusting. As a father with two daughters, I can't imagine what that mom is going through," Chicago's police chief Eddie Johnson said Tuesday.
Police found the girl Tuesday morning. She was reunited with her mother and taken to a children's hospital for treatment.
Authorities were investigating the incident as a criminal sexual assault, and told AFP on that no suspects were in custody.
Police would not confirm details of the video, citing the ongoing investigation. Facebook did not immediately respond to a request for comment.
A man identified as the girl's uncle told US media that the young men involved, some possibly under 18, were known in the local community.
"They are terrorizing that entire neighborhood," Reginald King told TV station WGN.
The live online video of the alleged gang rape attracted as many as 40 viewers at one time, but no one contacted police, according to reports. The video was later taken down, reportedly after police contacted Facebook.
The incident was the latest in which Chicago violence spilled over onto Facebook Live.
There have been two fatal shootings, including of a two-year-old boy, and the kidnapping and torture of an 18-year-old, all captured on live streaming video on the social networking site.
Chicago has been grappling with a surge in violent crime, with last year accounted for the highest number of shootings and murders in nearly two decades.
Much of the violence is concentrated in a few impoverished areas, including the area where the teenage girl lives.