A 15-year-old girl in Chicago was apparently sexually assaulted by five or six men or boys on Facebook Live, and none of the roughly 40 people who watched the live video reported the attack to police, authorities have said.

It is the second time in recent months that the Chicago police 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.

Police only learned of the latest alleged attack when the girl’s mother approached Supt Eddie Johnson on Monday afternoon as he was leaving a station in the Lawndale neighbourhood in the West Side, a police department spokesman said. She told him her daughter had been missing since Sunday and showed him screengrab photos of the alleged assault.

The spokesman said Johnson immediately ordered detectives to investigate and the department asked Facebook to take down the video, which it did.

The spokesman said detectives had found the girl and reunited her with her family. He said she had told detectives she knew at least one of her alleged attackers, but it remained unclear how well they knew each other. Investigators are questioning several people, but no one is considered a suspect yet and no arrests have been made.

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The spokesman 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. Investigators would have to subpoena Facebook to find out who they were and would need to “prove a nexus to criminal activity” to obtain such a subpoena, police said.

Andrea Saul, a spokeswoman for Facebook, said she had no specific comment on the Chicago incident but that the company took its “responsibility to keep people safe on Facebook very seriously”.

“Crimes like this are hideous and we do not allow that kind of content on Facebook,” she said.

Jeffrey Urdangen, a professor at Northwestern University’s law school and the director of the school’s Center for Criminal Defence, said it was not illegal to watch such a video or to not report it to the police. He said child pornography charges would not apply unless viewers had downloaded the video.