The body of Janet Murgatroyd, 20, was found in the River Ribble in Preston after a night out in 1996.
A man's manslaughter conviction for her death was quashed on appeal in 2004.
Lancashire Police's Det Ch Insp Gary Brooks said: "Twenty-one years on from Janet's brutal murder we remain committed to finding her killer."
He added: "Janet was a young woman with her whole life ahead of her. A student at the University of Central Lancashire and a part-time staff member with Lancashire Police, she had much to look forward to and enjoy.
"However, this was all taken away from her in a vicious attack, which we believe was sexually motivated.
"This was an opportunist offence by a very dangerous man who, despite the passage of time, still needs to be brought to justice."
The student had been shopping with a friend in Preston on 15 June 1996 before drinking in several pubs later that day.
She was seen leaving The Adelphi pub with a man before separating from him and was later spotted sleeping on Fishergate near Preston Railway Station.
Janet was subsequently seen fleeing from a man across Penwortham Bridge, by a taxi driver early on 16 June.
Two witnesses described hearing the sound of a female moaning and saw a man crouched down on the bank of the River Ribble. Detectives think this was Janet and her killer.
Her body was found later floating in the River Ribble.
The cause of death was drowning and head injuries, a post-mortem examination found.
New CCTV footage of a student walking down a street prior to her murder has been released on the 21st anniversary of her death.