Editor's Note: Three teenage girls have been charged in the attack. Read the latest update here.

A disturbing video that shows a group brutally beating up a teenage girl in Chicago has prompted a criminal investigation, police said. The victim, a 15-year-old, is seen getting punched by other teens while she screams.

A Chicago Police Department spokesperson, Officer Jose Jara, said during a news conference Wednesday the people who attacked the teen were people she knew.

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"At this point it is believed the victim was invited out by some girls who are so-called friends, who are eventually the offenders in these disgusting incidents," he said. Authorities said they are also investigating whether she was also sexually assaulted.

Stringer Harris, a local community activist who was speaking on behalf of the family, said the girl has a mental illness and wouldn't disclose anything further. He said her "friends" were taking her to an encounter with a "guy to have relations with," but the teenager was against that, which he said angered them.

Police Investigate Attack On Teen Girl Caught On Video Chicago police and a spokesperson for the victim's family discuss an investigation into a viral video showing several teens attacking a 15-year-old girl on the South Side. https://cbsloc.al/2y7XDTc Posted by CBS Chicago on Wednesday, July 24, 2019

"As she was walking through the alley, her friends came up to her and approached her and asked her what was she going to say to the police," he said. "At this time, she said 'nothing, I wasn't going to do anything, leave me alone, I just want to get on a bus and go.' One of her friends hauled her off and hit her in the face striking her in the head that left lacerations on her forehead."

In the now-viral video, the teen is cornered by at least five people and tries to defend herself. They're overheard laughing as she falls to the ground where the punches continued.

According to CBS Chicago, she disappeared after the attack on Saturday. She was found Tuesday, on the same day she was reported missing, after a tip on social media led to family and friends to find her on the train. The victim was taken to a hospital and released early Wednesday.

Police are trying to track down the teens involved in the attack as well as the person who filmed the entire thing.

"This is an ongoing investigation and information is coming in, but we need the public's help in keeping the offenders accountable," Jara said.

Editor's note: This story has been updated to remove the name of the teenage victim.