Chicago police are investigating an assault and possible sexual assault after a video claiming to show a brutal attack on a teenage girl with disabilities went viral this week.

The video shows a group of girls swarming and striking the girl repeatedly as onlookers laugh. The girl, who then falls to the ground, cries for help. The clip was published Monday night by an unidentified Twitter user who claimed to be a friend of the victim. The tweet said that the girl has a "mental disability."

The Twitter user did not immediately respond to USA TODAY's request for comment.

The 15-year-old victim was gone for five days before she was reported missing by her family on Tuesday, police spokesperson Jose Jara said at a news conference Wednesday. Jara said she was found by a concerned citizen and her father and was "discovered to be the victim of some sick and disgusting crimes."

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"At this point, it is believed that the victim was invited out by some girls who are so-called 'friends' who are eventually the offenders in these disgusting incidents," Jara said.

The group in the video offered to walk the victim to the subway as a ploy to jump her and videotape the attack, WLS-TV reported.

She was taken to a hospital where she was treated and released, Jara said, and she indicated to detectives she was sexually assaulted off-camera.

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Police spokesman Anthony Guglielmi said Tuesday on Twitter that the video "is beyond disturbing to watch and this young girl deserves far better."

The video has been forwarded to the Chicago Police Department's Special Victims Unit for further investigation, he said. Guglielmi did not immediately respond to a request for comment from USA TODAY.

One of the alleged female attackers, a student at Phillips Achievement Academy High School, originally published the video on Facebook.

Representatives for the high school did not immediately respond to USA TODAY's request for comment.

Contributing: N'dea Yancey-Bragg, USA TODAY; Associated Press

Follow Joshua Bote on Twitter: @joshua_bote