Christchurch police are looking for the drivers of two cars seen near where sex worker Mellory Manning was working last Thursday night.
Ms Manning was violently killed that night and dumped in the Avon river.
Detective Inspector Greg Williams says that after trawling through hours of CTV and security-camera footage of cars passing the corner where she was working, police have narrowed their interest down to two particular cars and want to speak to their drivers.
The first was a white vehicle described as being like a Honda; driven by an Indian man, it was in the area at about 10.15pm.
The other is a dark blue or black four-wheel-drive with tinted windows and highly polished wheels, seen around 10.30.
Appeal to public
But the general public's help is still needed to solve the case.
Mr Williams says someone may be holding crucial information without realising how important it is. They should contact the police if they have seen or heard anything out of the ordinary.
He says the person or people involved in Ms Manning's death may be taking an unusual interest in the case or acting in an unusual way.
They may have hidden or tried to sell the car in which the attack is believed to have occurred, or burnt clothing they were wearing.
Ms Manning was strangled, beaten and stabbed before her body was dumped. Her injuries show two weapons were used to kill her, a knife and possibly a length of reinforcing steel.
It is possible that more than one person killed her, given the use of two weapons, but the police cannot yet say for sure.