A boy thought to be as young as 11 has sexually assaulted a woman in a railway station booking hall.
The assailant laughed as he left the scene at Stockport and later returned and spat at rail staff.
British Transport Police is appealing for witnesses to the incidents which happened at about 3.45pm on Friday June 24.
Pc Rebecca Swift said: "The victim was waiting with her boyfriend in the booking hall of the station when she was inappropriately touched by a young boy who then left the station laughing.
"He returned a short while later and began messing around by the ticket barriers at the Edgeley side of the station. Station staff asked him to leave and he responded by spitting at them.
"The victim of the sexual assault was left feeling humiliated and shocked by what happened. A saliva sample was collected following the incident, and officers are now working to try to identify the boy.
"If you witnessed what happened it is vital we hear from you as any information could be crucial to our investigation.
"We take all reports of this nature very seriously and our Report It to Stop It campaign, which aims to combat unwanted sexual behaviour on the railway by encouraging members of the public to report it, is proving very successful in supporting victims and bringing perpetrators of these crimes to justice."
The boy is described as white, about 11, with short dark hair, and was wearing a black bubble jacket with a hood, black trousers and black trainers.
Anyone with information should contact British Transport Police on 0800 40 50 40 or text 61016 quoting reference number 171 of 28/06/2016.