Essex Police is hoping a group of students pictured on CCTV will come forward as key witnesses to an attempted rape in Chelmsford.
The attack happened when a 28-year-old woman was walking along New Street shortly before 2.30am on March 1, 2012.
Police officers are also investigating three further attacks in the town.
The second attack happened three hours later when a 36-year-old woman was waling along the Bunny Walks by the side of the nearby Riverside Retail Park. This is being treated as a sexual assault.
The third attack happened shortly after 2am on March 3 when a 28-year-old woman was standing outside Chelmsford Cathedral. This is being treated as a sexual assault.
Senior Investigating Officer, DI Tracey Harman, said: "We have examined hours of CCTV footage and these students saw the attacker in the hours leading up to the sexual assault under the railway
bridge on New Street.
"We would urge them to contact us as they all possibly can help us identify the man who lunged himself on the woman."
A 42-year-old man of no fixed address and a 25-year-old man from Chelmsford are currently on police bail until May 9, 2012 after having been arrested in connection with the three assaults.
Anyone with information in connection with these sexual assaults or is a student pictured in the CCTV image is urged to contact detectives at Brentwood's Sexual Offences Investigation Team on 101.