CHICAGO (CBS) — The Illinois Department of Children and Family Services is investigating allegations of neglect against the family of a 15-year-old girl with special needs who was the victim of a brutal beating caught on video. The agency also has taken the girl into protective custody.

Four girls have been charged in the attack, after police investigated video of the beating posted on social media. Three of the girls are charged with aggravated battery, and one is charged with mob action.

Police said the girl had been missing before she was attacked on Monday. Police learned of the video of the beating the next day, and hours later a friend found the girl on the Blue Line. She was taken to a police station, and then to a hospital, where she was treated and released.

DCFS said Thursday night that the agency has had prior contact with the family and received a hotline call involving allegations of neglect the same day the girl was found. The agency took protective custody of the girl the next day.

“The tragic incident is heartbreaking. The department is committed to protecting the state’s most vulnerable children. We are working closely with law enforcement to fully investigate what happened in this case,” DCFS spokesman Jassen Strokosch stated in an email.

Police have said the girl met with several teens who “she thought were her friends” on Monday, the day of the attack. Those friends taught her how to flash gang signs before the beating.

“It broke my heart to see this group of young people turn on her and escalate it into what we saw, a physical assault,” Chicago Police Supt. Eddie Johnson said. “There simply cannot be room for this type of divisive and hateful behavior.”

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

A family friend, community activist Stringer Harris, said the girls hit her in the face and caused lacerations on her forehead. Harris said the attackers were upset that she did not want to participate in a sex act they were planning and planned to tell the police about it.

Johnson did not specifically confirm that claim.

“There was some things going on that these young ladies wanted her to do that she didn’t do, but we’ll leave it at that for now,” Johnson said.

The victim’s grandmother said the girl’s attackers must face consequences.

“I’m mad that they took advantage of my baby like that,” she said “They need to get off the street. If they did it to her. They’ll do it to someone else.”

Police also have said they are investigating the possibility the victim was sexually assaulted during the time she was missing, based on statements she made to detectives.

One of the girls charged in the beating was due in Juvenile Court on Friday. The victim’s father said there also is a hearing in Juvenile Court related to the DCFS investigation.