A reward of up to £3,000 is being offered for information after a 21-year-old man was raped and robbed in Watford last year.
The sexual assault took place in Riverside Road and around the park area at approximately 3.30am as the victim walked home alone from a night out on June 22.
He was approached by two men who tried to take his mobile phone.
He attempted to get away from them but they forced him to perform a sexual act before demanding money from him and marching him to different cash machines in the town centre.
CCTV images from Wiggenhall Road and King Street have been released.
Both men are white, of a slim build around 6 feet tall, and have short hair.
One had been carrying a shovel which was used to threaten the victim. He was wearing a lightly padded black-hooded gillet with a yellow trim around the arms and a distinctive "New York" motif on the back.
He was also wearing a dark coloured t-shirt, light blue jeans and white trainers.
The other man was wearing a pastel checked short sleeved shirt, dark blue jeans and brown shoes.
Crimestoppers is offering a reward of up to £3,000 for information passed that leads to an arrest and conviction.
Ann Scott, Crimestoppers eastern regional manager, said: "This incident in the safe county of Hertfordshire is incredibly rare and disturbing.
"Nobody has anything to fear by contacting Crimestoppers if they have information about this offence or they recognize any of the individuals in the CCTV.
"We never ask for your name or take any personal details. We do not record or trace your call or IP address. You won’t have to give a statement, speak to the police or go to court - we only want to know what you know, not who you are. In the 26 years that Crimestoppers has been running we have never broken our promise of anonymity so nobody will know you contacted us."
Anyone with information on the crime can contact Crimestoppers anonymously on 0800 555 111 or through the anonymous online form.
Rewards will only be payable for information passed to Crimestoppers’ 0800 555 111 number.
Information passed to police or via the Crimestoppers online secure form will not qualify.