The alleged sexual assault of a 15-year-old girl by five or six males was streamed on Facebook Live, according to Chicago Police.
Around 40 people were said to have been watching the stream at one point but nobody reported the incident to police.
A police spokesman said authorities first learned of what happened after the girl’s mother approached the head of police as he was leaving a station.
Detectives have questioned several people but no arrests have been made.
The girl had been missing for a day but has now been reunited with her family.
The video has since been removed.
Supt Eddie Johnson was shown screenshots from the video by the girl’s mother.
He was said to be “visibly upset” after seeing the footage.
“Crimes like this are hideous and we do not allow that kind of content on Facebook,” a spokeswoman for the company said.
“We take our responsibility to keep people safe on Facebook very seriously and will remove videos that depict sexual assault and are shared to glorify violence.”
In January, Chicago police arrested four people following a separate incident in which a man’s alleged assault was live streamed, also on Facebook Live.