Several girls seen viciously beating a 15-year-old girl in a viral video were friends of the girl, who has a mental disability and had been missing for five days before she was found Tuesday, Chicago police said.

“She was discovered to be a victim of some sick and disgusting crimes that took place,” Chicago Police Officer Jose Jara said at a news conference Wednesday outside a police station at 51st Street and Wentworth Avenue.

“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.”

In the video, the 15-year-old can be seen walking with a group of people when a female in a black tank top and shorts walks behind the girl and punches her. Someone recording the girl starts laughing as more punches are thrown and the girl begins to scream and tries to protect herself.

Several girls then surround her and continue hitting her until she falls to the ground. Just before the video ends, someone appears to help the girl to her feet as she continues to cry.

WARNING: GRAPHIC VIDEO

“They beat up on a defenseless young lady who suffers from a mental illness,” said activist Stringer Harris, who was with the girl after she was found and has been in communication with the girl’s family. “ ... This is sad.”

Calls to family members were not returned Wednesday.

The girl, who is from the South Side, had been missing for five days before a relative reported her disappearance Tuesday, the same day she was found by her father and another person, police said. While police did not confirm the location of the attack, Harris and neighborhood residents said it took place in an alley near 80th or 81st Street and Exchange Avenue in South Chicago.

Police also did not say where she was found or why the girl had been missing for so long before authorities were called. Jara also did not give a motive but said she was beaten while she was missing.

Stringer said the girls in the video were mad at the victim because she refused to engage in a sex act in exchange for a tattoo.

“This lewd act that her friends wanted her to do was to get tattooed,” he told reporters. “Because she didn’t do this lewd act they were upset with her.” He also said the victim had attempted to alert a police officer in the area to what was going on, but the girls stopped her.

Jara said detectives are also looking into the possibility that the girl was sexually assaulted while she was missing “based on initial testimony from the victim herself.”

He added: “I don’t have all the information on it at this point, but that seems that’s the route it’s going. Due to the unique circumstances and the sensitive nature of these crimes ... the task of verifying the facts in this case will take some time.”

The girl was briefly hospitalized but was released early Wednesday. Stringer said she is now safe.

“She’s at home now with her father. She’s at home and she’s in high spirits,” he said.

Jara said police were searching for witnesses and any video evidence. No one was in custody, he said.

Residents who lived in the area said they knew some of the girls involved in the attack, and said one lived nearby where the attack took place.

Neighbor Julesha Davis said she thought the people seen in the video should be locked up.

”You shouldn’t do that to nobody,” Davis said, saying it shouldn’t matter if the girl has a disability or not.

Jalisa White, who said she talked to the sister of one of the girls involved in the fight, said she asked the sister, ”What if it was you?”

Jara said anyone with information should call Area Central detectives at (312) 747-8380.