The murder of a Devon schoolgirl who died in 1997 remains unsolved after all new leads were exhausted following a police appeal last November.
Kate Bushell, 14, was killed while walking her neighbour's dog.
She was found dead with knife wounds to her neck on 15 November 1997 in a field near Exeter by her father.
Investigating officers followed 10 possible leads after the 20th anniversary appeal but have not been able to find the killer.
Officers say the case, which is Devon and Cornwall Police's biggest ever murder investigation, costing more than £1m, will "always remain open".
Detectives remain convinced that the offender has a local connection due to the location of the crime scene, an isolated rural spot mainly used as a cut-though and by local dog walkers.
In June 2017 a group of serving and retired detectives began re-examining key evidence in the hope of finally catching her killer.
On the 20th anniversary of her death a new major public appeal was launched for people to come forward with critical and previously withheld information.
Within weeks of the new investigation in November more than 200 calls had been received by the investigating officers.
Former Det Supt Paul Burgan, of Devon and Cornwall Police, who led the review of the murder investigation, said he was "incredibly frustrated" that the force's highest-profile murder investigation remained unresolved.
"Unfortunately, despite the volume of calls we still do not know who is responsible for Kate's death," he said.
Kate's older brother Tim appealed to anyone who might be withholding information to speak out.
Crimestoppers has offered a £10,000 reward for information that leads to the arrest and conviction of the killer.