Lincolnshire Police had extra patrols in Lincoln and warned women not to walk alone at night after a sex attack in the city centre at the weekend.
A 19-year-old woman reported that she was sexually assaulted at around 6.30am on Saturday, January 18 on Tentercroft Street.
She had been walking near to the Tentercroft Street car park when a man approached and assaulted her.
Investigating officers spent the weekend trying to locate the man.
The suspect is described as a young white male, around or shorter than 5ft 7in in height, and was reported to be wearing blue jeans and a black puffa jacket.
DS Rachael Sadowski, who is leading the enquiry, said: “This was a really frightening experience for the woman involved and while not seriously physically hurt, she’s been left understandably shaken and upset.
“We’re offering her support and have a number of lines of enquiry ongoing to try and find the male responsible.
“I would like to ask that individuals, and women especially, are extra cautious in Lincoln at the moment.
“We’re not sure of the intent of this male and while we’re doing all we can to track him down it would be sensible for the time being if women made sure they weren’t walking alone during hours of darkness in areas where there aren’t a lot of people.
“Also just being aware of the people around you is a good idea, and that’s good advice at any time.
“If you have any information about this investigation, or witnessed anything that could help, please contact us as soon as possible.”
Inspector Gareth Whitelock said: “Officers will be carrying out extra patrols in the area as we seek to apprehend the suspect.”
Anyone with information can contact Lincolnshire Police on the non-emergency 101 number.