The witness, who asked not to be identified because she fears for her safety and her friend’s, said she was at a party at the Alpha Epsilon Pi fraternity’s Lambda Chapter house when another friend called her to the bathroom. She said someone had been pounding on the door, where her friend was locked inside, and that the lock was eventually picked.

The witness said she entered and was shoved. She told the man to get away from her friend and leave, then helped the woman get dressed and walk out, supporting her with another friend.

Afterward, she said, she saw the man with a group of friends who were laughing and cheering. She confronted him and chased him as he ran, according to her account. She described the experience as emotionally taxing and disturbing.

Chicago police are investigating the reported assault. The 21-year-old victim was taken to the University of Chicago Medical Center for treatment, officials said. The university said the victim was not affiliated with the school and had been attending a party at the house, which is near campus.

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

Alpha Epsilon Pi’s national organization said it is cooperating with the investigation and directed the local chapter not to hold events or programs while the inquiry is pending. The chapter said it condemns the reported assault, that the suspect was chased from the house, and that it is working with police. It also said no one affiliated with the fraternity was involved or complicit.

The witness said she and the alleged victim have received online threats since the incident. She said the threats have heightened her concern for both of their safety.