A woman was sexually assaulted as she was walking alone at night in Northampton.
The attack happened in Earl Street, The Mounts, when an unknown man approached the victim and tried to start a conversation with her.
He then pushed her to the ground near some bushes and pulled at her clothing at which point the victim screamed for help and the man ran off.
The attacker was white and aged in his late 20s or early 30s. He was 6ft with a medium build and was wearing a grey-coloured top with a hood.
Detectives are particularly keen to speak to a man aged in his late teens or early 20s who may have tried to intervene while the man was following her.
He is believed to have been part of a group of five or six people gathered in Earl Street moments before the assault.
They are not suspected of being involved in the incident.
Anyone with information about the incident, which happened between 3.30am and 4.30am on Sunday, February 26, should call Northamptonshire Police on 101 or Crimestoppers, in confidence, on 0800 555111.
Police have released the information today (Thursday).