The third inquiry into the murder of 23-year-old backpacker is being delayed by red tape, claim Dyfed-Powys Police.
Kirsty Jones was three months into a round-the-world trip
Kirsty Jones, from near Brecon, Powys, was raped and strangled at a guest house in north Thailand in 2000.
Last year, prosecutors in Thailand agreed to launch a fresh investigation into Miss Jones's murder on the fifth anniversary of her death.
But police said Thai officials had yet to release papers which would allow them to question witnesses in the UK.
Miss Jones' mother Sue said the situation was "very, very frustrating".
Miss Jones, a Liverpool University graduate, was found dead in a room at the £1-a-night Aree guest house in Chiang Mai, more than 400 miles north of the capital Bangkok.
She was three months into a two-year round-the-world trip when she was murdered on 10 August, 2000.
Since then there have been several arrests in the investigation, but none have led to a conviction.
In the past, the Royal Thai Police have been criticised for their handling of the case.
However, the latest inquiry has been placed in the hands of Thailand's FBI, known as the DSI - the Department of Special Investigations.
But Dyfed-Powys Chief Inspector Steve Hughson said before detectives could interview British witnesses, they required an official document otherwise evidence would not be valid in court.
The force helped the Thai authorities to secure the killer's DNA, and British witnesses who were in Thailand at the time of Miss Jones' death could hold clues to his identity.
Kirsty Jones's mother Sue visited the murder scene last year
"The Thai police are interested in asking witnesses questions, but before we can do this on their behalf we need the proper authority which is a request form from the Attorney General in Thailand," he said.
"We need the authority so the interviews are on a proper legal footing."
Ch Insp Hughson added: "We share the family's frustrations at the time delays."
Sue Jones said: " We are in their (Thai authorities) hands until we receive this official document. We have been waiting for 12 months for it, but every time we ask for it they tend to delay things."
The Thai authorities have been asked to comment.