Disturbing video shows girls jumping, beating 15-year-old in Chicago

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A shocking video posted to Twitter on Monday shows several girls in Chicago jumping and beating a 15-year-old girl who has a "mental illness," according to a friend of the victim.

The friend alleges the victim needed help walking to the nearest Red Line station and the other girls offered to help. However, on the way, the victim was jumped for allegedly "calling the police on them."

A criminal investigation has now been opened after Chicago police were made aware of the incident. 

Officer Jose Jara, a police spokesman, said Wednesday that detectives are trying to piece together basic information, including where and when the attack caught on video occurred.

"This is beyond disturbing to watch and this young girl deserves far better," Chicago Police spokesman Anthony Guglielmi tweeted. "This clip has been forwarded to the #ChicagoPolice Special Victims Unit and a criminal investigation has been opened."

The Associated Press contributed to this report.