Deahvion Austin was last seen wearing a pink hoody and green sweater in the 1400 block of South Avers, according to Chicago Police.
Police said Austin is described as a 5-foot-7, 146-pound black girl, who wears her hair naturally.
The girl was later reunited with her family after being located by detectives, Guglielmi tweeted Tuesday, adding that authorities are conducting interviews. "I just want to make sure she's ok cause i know she's probably scared and embarrassed".
The girl's mother said she was alerted on Monday morning by the girl's uncle to a video captured on Facebook Live that showed the teenager being sexually assaulted by several young men, news station WGN said.
Jeffrey Urdangen, a professor at Northwestern University's law school and the director of the school's Center for Criminal Defense, said it isn't illegal to watch such a video or to not report it to the police. She told him her daughter had been missing since Sunday and showed him screen grab photos of the alleged assault.
About 40 people reportedly watched the video, but no one reported the incident to police, according to the AP.
He said Johnson immediately ordered detectives to investigate and the department asked Facebook to take down the video, which it did.
They are treating the case as a sexual assault.
"Here's what's even more disturbing, more than the fact that they did this, there were so many people that saw it and didn't pick up the phone to dial 911", Chicago Police Superintendent Eddie Johnson told WGN-TV. "That's just not right and [we're] working on it and trying to bring it to a successful resolution".
This is the second time in a few months that a Chicago crime was live streamed over Facebook.
Facebook began airing its Live feature a year ago.