A POLICE whistleblower has claimed he was told to withhold potential evidence by fellow officers during the investigation into one of Scotland's most famous unsolved murders.

Aksoy Ozer, a former constable who worked as a translator in the case of murdered prostitute Emma Caldwell, gave a statement to Strathclyde Police in 2007 alleging misconduct by officers in two forces.

He alleged that he was asked to lie about his use of audio equipment in the probe, and claimed he was warned against speaking to his doctor about the case after saying he felt under "immense pressure".

Ozer added that he spoke Turkish - the language of the original suspects - on a "very limited basis" and held "no formal qualifications in translating".

The case against the Turkish men collapsed and Police Scotland is now preparing to re-investigate a lead regarding a Scottish suspect described as an "obsessive user" of prostitutes.

Highlands MSP John Finnie said the Ozer allegations had to be investigated "as a matter of urgency" by officers unconnected to the original murder case.

Caldwell, a 27 year old heroin addict who was working as a street prostitute to feed her addiction, was found dead in woods near Biggar in 2005. She was one of eight prostitutes murdered in Glasgow between 1991 and 2005. The murders created fear that a serial killer was on the loose, and trawling Glasgow's redlight area for victims.

Emma Caldwell's murder led to one of Strathclyde Police's biggest murder investigations and thousands of witness statements were taken. However, ten years on, no-one has been brought to justice.

In the original investigation, officers believed that several Turkish men were linked to Caldwell's death and set up a surveillance operation in a cafe in Glasgow used by the then suspects.

The covert activity produced what police believed at the time were incriminating translations and four Turkish men were arrested in August 2007.

However, experts challenged the translations and the case never went to trial. One of the consultants, Oxford-based academic Professor Kerem Oktem, had been hired by Strathclyde police to carry out a review.

He said: "We concluded that based on the material we were provided, the recordings, that it was not possible to make any conclusive statement about their involvement in the murder. It was simply not possible based on the material in the recordings."

It has since emerged that officers on the case had downplayed a local, non-Turkish suspect, who is alleged to have been an "obsessive user of prostitutes" with a "very violent history".

This suspect also admitted in police interviews to taking Caldwell to the murder site on previous occasions.

A Sunday Herald investigation can now reveal internal allegations of police misconduct during the failed probe into the Turkish men.

Ozer, born in Edinburgh to Turkish parents, had been seconded to work on the investigation from Grampian Police in early 2006 to help on the surveillance side.

He and other officers who had knowledge of the Turkish language were drafted in to translate recorded conversations and produce transcripts.

In November 2007, weeks after the Turkish men had been arrested, Ozer gave the Strathclyde force a written statement about the alleged behaviour of other officers.

He wrote how, eight months earlier, in "preparation for arrests", he was instructed to download specific conversations involving the suspects onto a hard disc.

These discs, he wrote, were to be used for interviewing the suspects.

He stated: "[An officer] then went on to tell me that I was to ensure that in any references to this downloading that I had to state that [officer] was present when I did so as I was not fully trained in the use of the programme. This [officer] was not present when I used this programme."

When Ozer began to use the downloading equipment, he wrote that he picked up "more information" than before: "I went onto explain that in using this equipment I had found errors in my previous transcripts. I offered to go over all the transcripts using this new equipment. I was told not to as the case was already with the procurator fiscal. I was further told to just write down any additional information and not to officially record it as before."

Ozer continued downloading and found yet more information relating to the case, a discovery he said did not impress a colleague: "[The officer] immediately asked why this had not come out previously and I explained the use of the new equipment. [The officer] then told me 'we could have a problem here, if this came out in a trial then we would be questioned as to why we never listened to all the discs using the new equipment'."

"[The officer] then told me not to mention this and to withhold this from the trial. I was extremely concerned regarding this as I had just been told to lie in court."

Within weeks of making the statement, the Turkish suspects were released on bail and the case was dropped in mid-2008.

Ozer also provided details of a meeting with officers in 2007, in which he informed those present that he wanted an appointment with his doctor due to feeling "immense pressure".

He wrote: "At this point, [officer] ordered me that I could not discuss matters with my doctor. This took me aback."

Ozer also wrote that, after receiving an assurance he was off the inquiry, an officer told him that this was not the case: "[Officer] told me that [officer] had decided that I would remain in the case but would be described as an advisor and that I would be required to submit a statement to that effect."

In addition, he stated that he was "not trained" to use a specialist listening device and said cassettes and logs of sensitive materials were located in an "insecure filing cabinet".

Ozer, who has dual British-Turkish nationality, also wrote: "I am not 100% fluent in the language and only speak on a very limited basis. I have little or no knowledge of the Kurdish language. I have no education on the Turkish language and I hold no formal qualifications in translating."

He flagged up the misconduct allegations to the then Lord Advocate Elish Angiolini in a letter in 2008 and lodged two grievances with the police.

In his letter to Angiolini, he wrote: "Could you please confirm if The Crown Office have investigated or instructed an investigation into my allegations?"

The Crown Office wrote that the Professional Standards Unit at Strathclyde Police had been asked to look at matters raised by Ozer.

In 2009, Ozer's representatives from SEMPER Scotland (Supporting Ethnic Minority Police staff for Equality in Race) wrote to Alex Salmond, at that point the police officer's constituency MSP, in which he mentioned the witness statement.

The representative stated that Ozer believed he had been "made the scapegoat for the failure of the investigation". He quit the police in 2010.

Finnie, who was a police officer before becoming an MSP, said if Ozer was "told to withhold information from any trial and instructed not to share his growing concerns with his GP then the public need assurances that such malpractice has been or will be rooted out.

"These matters constitute a potential attempt to pervert the course of justice and must be examined, as a matter of urgency, by senior police officers with no connection to the former Strathclyde Police or this enquiry and be overseen by Crown Office officials likewise unconnected to the original case.

"The obligation on the police is to provide all evidence to the Crown Office Procurator Fiscal Service, not simply selective extracts which purport to support any predetermined view as to the accused."

Police Scotland Chief Superintendent Ellie Mitchell of Professional Standards said: "A number of internal issues were raised, inquiries carried out and disposals made."

Assistant Chief Constable Malcolm Graham said of the reinvestigation: "Police Scotland's priority is to focus on undertaking a reinvestigation of the murder of Emma Caldwell. We are at an advanced stage in preparations to conduct that.

"It is hoped that advances in investigative techniques and forensics will assist in the investigation. All necessary resources with the appropriate skills and experience will be deployed to ensure a thorough investigation takes place.

"Senior officers from Police Scotland will be meeting with the family to outline the plans already in place and provide further information on the areas to be explored in an attempt to identify new evidential opportunities."

A Crown Office spokesperson said: "Following a complaint against the police a report was submitted to the Procurator Fiscal.

"After careful consideration of all the facts and circumstances, Crown Counsel concluded that there was insufficient admissible evidence and there should be no action in relation to this matter.

"Crown Counsel's decision was communicated to Mr Ozer in October 2010."

Ozer declined to comment.