Chicago police were working Wednesday to identify whoever was involved in the apparent gang-rape of a 15-year-old girl that was streamed live for the world to watch on Facebook.

Interviews are ongoing but no formal suspects have been named, according to police spokesman Anthony Guglielmi. On Twitter, he denied reports that a suspect had been arrested.

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Guglielmi added that investigators were working to build a criminal case; no charges have been announced.

Police first learned of the attack after the girl’s mother approached Superintendent Eddie Johnson late Monday afternoon and showed him screenshots of the alleged assault. The teenager had been reported missing Sunday night after failing to return home from the store.

The horrifying attack was made even more disturbing because as many as 40 people watched the live feed showing up to six people sexually assaulted the teen and did not report it to the police.

The girl’s mother Stacey Elkins told WLS that the look on her daughter’s face when she saw the video was “just pure fear.”

“It was very, very graphic. She’s pulled toward the bed,” the girl’s uncle Reginald King, who first spotted the video, told the Chicago Tribune. “To have it put out there like that, publicly. It’s not right.”

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He added: “It hurts me to my core because I was one of the last people to see her. I want to make sure this never happens to anybody else’s kids, and if that starts with taking down this one group, I’ll make that my life’s mission.”

King said a teenager alerted him of the assault. “This is one of the bravest things I’ve ever seen a kid do. There were adults who saw this. None of them had the wherewithal to day, ‘Hey, I gotta call someone.’”

The teenager was found walking about four blocks from her home on Tuesday and taken to the hospital for treatment.

According to the Chicago Sun-Times, the girl is a student at Lane College Prep High School, a selective enrollment magnet school. She had been reported missing for a few days last November before being safely reunited with her family.

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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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."

She added, "Crimes like this are hideous and we do not allow that kind of content on Facebook."

Alderman Mike Scott, whose ward includes Lawndale on the city’s West Side, told the Sun-Times he was baffled by the Facebook Live post.

“Of course, I didn’t grow up with social media. But it’s becoming a place where young people act out movie scenes if you will, people are getting shot and killed and beaten on Facebook Live,” he said. “I really don’t understand the fascination or the glorification of this violence. People want to get Facebook and Instagram famous, not knowing the consequences.”

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.

This is the second time in months that Chicago police have investigated an apparent attack streamed live on Facebook.

In January, four people were arrested after cellphone footage streamed live on Facebook showed them allegedly taunting and beating a mentally disabled man.

The suspects in that case -- Jordan Hill, 18, Tesfaye Cooper, 18, Brittany Covington, 18, and Tanishia Covington, 24 – were charged with hate crime and battery for allegedly torturing the white man for as long as 48 hours, while shouting racial and anti-Donald Trump slurs.

The Associated Press contributed to this report.