A woman was raped at the popular Crystal Bar in Sheffield city centre.
The 29-year-old victim was sexually assaulted by an unknown person at the Carver Street venue in the early hours of Sunday, May 7.
Police have been trawling through CCTV but have not found a suspect. They are now appealing for any witnesses who may have seen someone acting suspiciously to contact officers.
Detective constable Jon Kirkham said: “Since police received the allegation, we have been working closely with the victim and the nightclub to collect as much information as we can about what happened that evening.
“This had included meticulously reviewing CCTV footage from inside and outside Crystal, in an attempt to piece together the victim’s movements and also identify any suspicious activity.
“The victim is receiving support from officers and at this time is unable to provide any further information about who may have been responsible for this incident.
“We now need the public’s help to gather information about any suspicious activity they may have witnessed in Crystal that night, between 12.45am and 1.45am on Sunday 7 May. I’d also be keen to hear from anyone who saw anything suspicious in the surrounding area at similar times.
"Were you in Crystal that night? Did you see anything that could help officers?"
Contact police on 101 quoting incident number 179 of 7 May 2017.