CHICAGO (CBS) — The 25-second video is painful even to watch – a teenage girl screams piercingly, then flinches, as several other teens punch her in the face and back and even laugh.

A criminal investigation has now been launched since CBS 2’s Dana Kozlov sent police the now-viral video of the attack.

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In the video, the girl, believed to be 15, is seen trying to defend herself as another girl continues to corner her and hit her – with the help of others.

Those who know the young victim say it happened Saturday near 81st Street and Exchange Avenue in the South Chicago neighborhood.

The girl disappeared after the attack, and a missing person’s report was filed. She was found Tuesday on the CTA Blue Line at Harrison and Kedzie. A tip on social media led friends and family to find her on the Blue Line.

She was checked out by doctors at Comer Children’s Hospital, and has been reunited with her family.

CBS 2’s Kozlov watched the video of the attack, which was posted on social media, with Dr. Mitchell Glaser.

“There’s someone yelling, ‘Beat her!’ like an adult in the back sounds like yelling, ‘Beat her down!’” Glaser said.

Glaser is the chief of adolescent psychiatry at AMITA Health Saints Mary and Elizabeth Medical Center.

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“When I started in child psychiatry 20-something years ago, the stereotypical patient was an angry, aggressive boy,” Glaser said. “I’d say almost maybe the opposite now – the stereotypical patient is an angry, aggressive girl.”

In fact, Glaser says the treatment program for teenage girls at his hospital is now double the size of the program for teenage boys.

“I have not seen a study, so I can’t say definitively what the answer is,” he said. “From how I’ve seen it, I saw an increase in aggression and anger before social media became as prevalent as it is now.”

CBS 2 helped bring the video to the attention of Chicago Police. A representative first said police found the victim’s missing persons’ report, but not one for battery.

Police chief Communications Officer Anthony Guglielmi later tweeted: “This is beyond disturbing to watch and this young girl deserves far better. This clip has been forwarded to the #ChicagoPolice Special Victims Unit and a criminal investigation has been opened.”

CBS 2 has learned the teen’s name, but we will not be identifying her because she is a victim.

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Area Central detectives were handling the investigation Tuesday night.