CHICAGO (AP) — A 15-year-old Chicago girl 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 said Tuesday.

The video marks the second time in recent months that the Chicago Police Department has investigated an apparent attack that was streamed live on Facebook. In January, four people were arrested after a cellphone 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 the head of the police department, Superintendent Eddie Johnson, late Monday afternoon as he was leaving a department station in the Lawndale neighborhood on the city’s West Side, 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.

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

Guglielmi said Tuesday that detectives found the girl and reunited her with her family. He said she told detectives that she knows at least one of her alleged attackers, but it remained unclear how well they knew each other. He 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 by email.

A spokeswoman for Facebook, Andrea Saul, said she had no specific comment on the Chicago incident but that the company takes 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 Defense, said it isn’t illegal to watch such a video or to not report it to the police. He also said child pornography charges wouldn’t apply unless viewers were downloading the video.