Police in Chicago are searching for five or six men or boys suspected of sexually assaulting a 15-year-old girl on Facebook Live.
Spokesman Anthony Guglielmi said detectives had found the girl and reunited her with her family after she went missing on Sunday.
Mr Guglielmi said about 40 people viewed the attack on Facebook while it was happening, but none of them reported it to police.
He said Superintendent Eddie Johnson was leaving a station in the Lawndale neighbourhood on Chicago's West Side late on Monday afternoon when a woman approached him with photographs of her daughter apparently being sexually assaulted on Facebook.
He said Mr Johnson immediately ordered an investigation and the department contacted Facebook to take down the video, which it did.
It is the second time in months that the department has investigated an apparent attack streamed live on Facebook.
In January, four people were arrested after mobile phone footage showed them allegedly taunting and beating a mentally disabled man.