A woman was sexually assaulted and then had her phone stolen on a rail replacement bus near Watford.
British Transport police said the 28-year-old victim was sat on her own on a single decker bus replacing the Euston to Milton Keynes service in the early hours of Saturday, August 16, when the assault happened.
PC Chris Woolley, leading the investigation, said: "The victim was sat on her own on the single-decker bus. As it left Watford station, at approximately 1am, a man sat next to her and tried to engage her in conversation.
"As she attempted to move away he sexually assaulted her. She sat elsewhere on the bus for the remainder of the journey.
"When the bus arrived at Hemel Hempstead rail station, the woman realised her mobile phone was missing. She confronted the man but he denied taking it."
The suspect is described as a British Asian, possibly of Pakistani descent, in his early to mid 20s, 5ft 6in tall, clean shaven, with slight build, and spoke with a London accent.
At the time he was wearing a dark hooded top, dark jeans and black trainers. He was later seen sitting with a white man.
Officers now hope to hear from other passengers who were on the bus and may have witnessed the incident.
PC Woolley added: "I would like to hear from anyone who believes they were on that bus. They may have crucial information which will allow us to find this man and bring him to justice. Unwanted sexual behaviour of any kind will not be tolerated and we will always fully investigate all cases reported to us."
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference MSUB/B7 of 28/08/2014. Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555 111.