The witness, who requested anonymity because she fears for her safety and her friend’s, said she was at a party at the Lambda Chapter house of Alpha Epsilon Pi when another friend called her over. She was told that the 21-year-old victim was locked in a bathroom with a man.

The witness said she and others picked the lock and found the man naked and the victim on the floor with some of her clothes removed. She said the group outside and the man ran away after she confronted him.

“I made him get off of her and told him like to leave and to get away,” she said. The witness said she helped her friend get dressed and walk out, then chased the man and flagged down police.

University officials said in a message to students that the victim is not affiliated with the school. She was taken to the University of Chicago Medical Center for treatment, according to information provided by the university.

Chicago police are leading the investigation. University officials said a person of interest was identified Tuesday and barred from school grounds; the individual is not a U of C student and is not affiliated with the university. The school has not publicly identified the person, saying it would not do so until charges are filed.

The fraternity’s national organization said it is cooperating with the investigation and directed the chapter not to hold events or programs while the inquiry is pending. The local chapter said it condemns sexual violence, that the alleged suspect fled the house and was captured on cameras, and that members assisted the victim and are working with police. The chapter said no one affiliated with it was involved or complicit.

The witness said she and her friend have since received online threats. She described the aftermath as “emotional, taxing, alarming, disturbing” and said she is worried about their safety.