It occurred between 4pm and 5.30pm on Friday 24 October 2014, at a bus stop near the Accident and Emergency Unit at the Friarage Hospital, when an 18-year-old woman from Northallerton was assaulted.
An 18-year-old man from Richmond who was arrested in connection with the incident has been released on bail to allow further police enquiries.
During the assault a woman walking a dog spoke with the victim and a man called a security guard to deal with the incident. Officers need to speak to these two people as soon as possible.
Detective Constable Gillian Gowling, of Northallerton Protecting Vulnerable People unit, said: I urge these two potentially vital witnesses to come forward straight away as they may have important information that can help the investigation.
The victim who was left extremely upset by the experience is continuing to receive support from specially trained police officers.
I also need to speak to anyone else who was in the area at the time and has not yet already contacted the police, to do so as soon as possible.
Anyone who can help with this enquiry should contact Northallerton police straight away, select option 2 and ask for Gillian Gowling. Alternatively, information can be emailed to email@example.com
Or call Crimestoppers anonymously on 0800 555111, quoting reference number 12140184287.