The girl's mother was shown the screenshots and was able to identify her daughter, the Tribune reported.
Police have said that the attack involved five or six people, and that 40 people watched the assault live on Facebook and did not call the authorities.
A 14-year-old boy has been arrested after a missing schoolgirl was seen on Facebook Live being sexually assaulted by a group of young men. He says more arrests are expected.
"CPD arrested 1st of several juvenile offenders in Facebook sexual assault incident", police department spokesman Anthony Gugliemi wrote on Twitter late Saturday.
"From there, she was not allowed to leave and she didn't consent to what occurred", Deenihan said, adding that the victim knew one of the suspects.
Cops were not aware of the mid-March attack until the girl's mom, Stacey Elkins, walked up to Chicago police Superintendent Eddie Johnson as he was leaving a police station and showed him the shocking video.
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"Last month, a pregnant woman was recording herself on Facebook Live when a gunman opened fire, wounding her and killing a 2-year-old and a man".
"They humiliated themselves, they humiliated their families, and now they are going to be held accountable for what they did", Johnson said. "It makes you wonder".
The girl's mother told the Associated Press that her daughter received online threats following the attack and that neighborhood kids were ringing her doorbell looking for the girl.
The Post typically does not name victims of sexual assault.
The video marks the second time in recent months that Chicago police have investigated an apparent attack that was broadcast live on Facebook.