CHICAGO (CBS) — Four girls were arrested Thursday in what police described as “sick and disgusting crimes,” after a group of teens was caught on video beating a 15-year-old girl with special needs.

Police Lt. Ozzie Valdez, the acting commander of Area Central, said earlier Thursday that three girls – ages 13, 14, and 15 – were charged in the attack. Two of the girls were charged with aggravated battery and the other was charged with mob action.

A fourth girl, age 15, was also arrested Thursday afternoon and charged in connection with the attack, police said. Police did not release the specific charges against the fourth girl.

The charges come as CBS 2 has learned the Illinois Department of Children and Family Services is also involved.

Police said the teens who attacked the girl knew the victim before the attack, and she believed they were her friends.

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

Video of the beating was posted on social media, and Chicago police began investigating after CBS 2 sent them a link to the video.

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

A family friend has said the girls who attacked the victim were upset that she did not want to participate in a sex act they were planning and that was going to report it to police.

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 Thursday afternoon at police headquarters.

Police have said they also are investigating whether the girl was sexually assaulted, based on her statements to detectives, but Johnson would not discuss that investigation.

“We’re not prepared to talk about that at this time,” he said.

Meanwhile, the girl’s father spoke out on Thursday, as CBS 2’s Dana Kozlov reported.

“I’m hopeful, right, thankful, right, that my daughter has been found,” he said. “And her condition, right, I hope that it improves.”

CBS 2 is not identifying the girl’s father or other relatives, to shield her identity because she is a victim, possibly of a sexual assault.

The girl’s father wanted to thank those who helped his daughter. He has also acknowledged that there is now a DCFS investigation into his family.

“At the moment, she’s in protective custody, and whatnot, for her own safety and well-being,” he said, “and that’s it.”

In a statement, DCFS officials also revealed that the department has had prior contact with the family. The reason is something no one wanted to answer.

“Because there is an investigation still that’s going,” said a man with the girl’s father.

The girl’s father reported her missing the day after the attack. Hours later police learned about the video and launched a criminal investigation.

“If you’re dumb enough to post a crime on social media, that just shows you where your head is,” Johnson said.

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.

“One of her friends hit her in the face, striking her in the head, that left lacerations on her forehead,” community activist and family friend Stringer Harris said on Wednesday. “Another friend attacked her brutally and beat her and another gentleman was there as well.”

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.”

Three of the girls who were charged in the case turned themselves in to police with their parents Wednesday night. Johnson suggested they also provided police with information about others seen in the video.

“Some of the parents of the offenders and the offenders themselves also did the right thing and brought information and individuals forward to police,” Johnson said.

Valdez confirmed detectives are investigating other people involved in the attack but would not say how many.

On Wednesday, police said the girl had been missing for five days before her family filed a report, but police have not explained why the girl’s family waited so long to file a report.

The girl was located Tuesday evening, when a friend found her on the on the CTA Blue Line at Kedzie Avenue and the Eisenhower Expressway. The friend contacted the teen’s family, who took the 15-year-old to the police station. She later was treated and released at University of Chicago Medicine Comer Children’s Hospital.

The Department of Children and Family Services issued the following statement regarding the girl:

“DCFS is investigating allegations of neglect involving the 15-year-old with special needs that was brutally attacked this week. 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 opened the investigation following a hotline call on July 23 and took protective custody of the 15-year-old on July 24. The department has had prior contact with the family.”

The girl’s father says there is a court date related to one of her cases on Friday.