Police are offering a $100,000 reward for information about the murder of Ashburton teenager Kirsty Bentley in 1998.
Kirsty was last seen walking her dog on the afternoon of December 31, 1998. Her body was found in the Rakaia Gorge more than two weeks later.
On Monday detective inspector Greg Murton said police are now offering a cash reward for information or evidence leading to the identity and conviction of those responsible for her murder.
"Police have gone to great lengths to explore all possible scenarios and investigative leads and, even after more than 20 years, we remain committed to holding the offender or offenders to account," detective inspector Murton said.
"Our ultimate goal is to provide answers and a sense of closure for Kirsty's family. That's why today we're offering up to $100,000 for material information or evidence leading to the identity and conviction of the person or people responsible for her death.
"Someone out there knows the truth."
Murton urged those responsible to come forward - "if not for themselves, then for Kirsty and all those who loved her".
Murton also said police would consider immunity from prosecution for any accomplice who wasn't the main offender. The offer will remain in place for six months.
The Commissioner of Police will determine the amount of the reward and will, if necessary, apportion payment where there is more than one claimant.
Anyone with information is asked to contact Police via 105 and reference Operation Kirsty.
Alternatively, you can call Crime Stoppers anonymously on 0800 555 111.
On the day of her disappearance, Bentley met with a friend at Ashburton library at around 10:30am. The pair then went shopping and had lunch at McDonald's before Bentley was dropped home in the afternoon.
The teen then decided to take the family's black labrador for a walk - this was the last time she was seen alive.
When her mother arrived home that night and she had not returned she searched the route she would normally take twice, before telling Bentley's father who called the police.
Police initiated a search but couldn't find Bentley or the family's dog. The next day during another search the dog was found tied to a tree by the Ashburton River along with a pair of underwear and boxer shorts which were later confirmed to be Bentley's
Searches for the teen continued over the next 16 days until her body was found in the Camp Gully area of Rakaia on January 17.
