A "Good Samaritan" who came to the aid of a 16-year-old who had been sexually assaulted in Cambridge is being sought by police.
The girl was waiting for a friend in Drummer Street bus station on the evening of 1 October when she was grabbed around the throat by a man.
He pushed her into a fence and sexually assaulted her before being chased off by a passing woman.
Officers want to talk to the woman who they believe may have crucial evidence.
The offender was wearing a dark hooded top, black jacket and jeans.
Det Con Ellie Mansfield said: "This was a frightening incident for the victim who has been left shaken.
"We are keen to speak to the woman who chased off the attacker as she may have crucial information to help the investigation."