US police are searching for five or six men or boys involved in sexually assaulting a 15-year-old girl in Chicago and posting the act on Facebook Live.

About 40 people watched the live video but none of them reported the attack to police, authorities said on Tuesday.

Police only learned of the assault when the girl's mother approached the head of the police department, Superintendent Eddie Johnson, late on Monday afternoon as he was leaving a police station, department spokesman Anthony Guglielmi said.

She told him her daughter had been missing since Sunday and showed him screen grab photos of the alleged assault.

"For you to do something like that and then post it up on Facebook like it's a joke … it's just terrible," the girl's mother told news outlet WMAQ.

The girl's uncle told the Chicago Tribune that a teenager alerted him to the assault on Facebook Live.

Guglielmi said Chicago police asked Facebook to take down the video, which it did.

He said detectives found the girl on Tuesday morning. She was brought to a hospital where she was reunited with her family.

She told detectives that she knows at least one of her alleged attackers, the spokesman said, but it remained unclear how well they knew each other.

Several people questioned

Guglielmi said investigators are questioning several people, but no one is considered a suspect yet and no arrests have been made.

He said Johnson was "visibly upset" after he watched the video, both by its content and the fact that there were "40 or so live viewers and no one thought to call authorities".

Investigators know the number of viewers because the count was posted with the video. To find out who they were, though, investigators would have to subpoena Facebook and would need to "prove a nexus to criminal activity" to obtain such a subpoena, Guglielmi said.

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Jeffrey Urdangen, a professor at Northwestern University's law school and the director of the school's Center for Criminal Defense, told the AP news agency it is not illegal to watch such a video or to not report it to the police. He also said child pornography charges would not apply unless viewers were downloading the video.

"Crimes like this are hideous and we do not allow that kind of content on Facebook," the company said in a statement.

"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."

Users can flag offensive videos Facebook. One such report sends the content to be reviewed by a team operating 24/7 that can view content whether it is public or privately shared.

The video marks the second time in recent months that the Chicago Police Department has investigated an incident streamed live on Facebook. In January, four people were arrested after mobile phone footage showed them allegedly taunting and beating a mentally disabled man.

The suspects in that case have been charged with kidnapping, assault and hate crimes.