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When approached by Calendar, a spokesman said: "North Yorkshire Police invited the National Policing Improvement Agency to conduct a second voluntary review of the investigation in 2010.
"Their report concluded that the investigation had been conducted with integrity and objectivity and that, where individuals had been subject to investigation, no investigative opportunities had been overlooked.
"The reviewers noted that good practice was evident in the investigation and that those strategies reviewed conformed to nationally approved standards."
See the interview on Calendar tonight at 6pm.
As the inquiry into Claudia's disappearance enters its fifth year, they have called for a new inquiry with a wider remit. But, they say even that may be too late and whoever is responsible for Claudia's disappearance may have got away with it.
The mother and sister of missing Claudia Lawrence have strongly criticised the police investigation, saying a decision to focus on her private life may have cost vital clues.
Monday marks the four year anniversary since Claudia, a chef at York University, disappeared without trace.
In a frank interview Joan and Ali told Christine of their fears that the police closed their minds to other options, dismissing evidence that didn't fit with their theory.
Claudia's mother also accuses North Yorkshire Police of being rude and insensitive in their dealings with her and other potential witnesses.