POLICE Scotland have revealed for the first time that only three people gave witness statements during their year-long probe into allegations of election fraud in the Scottish independence referendum and they were all "subjects of the investigation".

Scottish Conservative leader Ruth Davidson, who gave a statement after being interviewed by the police, has consistently insisted that she was spoken to by police merely as a 'witness'.

A spokesman for the Scottish Conservatives would only say in response to the latest development: "Ruth was only ever interviewed as a witness."

Police say the three people were interviewed in connection with the investigation into allegations of electoral law breaches and had a "dual role" as both subjects of the investigation and as witnesses.

Davidson was interviewed by police after the Crown Office instructed that there should be an investigation after she said on a television that postal vote "tallies" were being taken in the weeks before the referendum ballot closed at 10pm on September 18, 2014.

After a year-long probe, police announced that "no criminality has been uncovered", but declined to explain the rationale for their assessment.

Police made initial telephone contact with Davidson before interviewing her at her parliamentary offices in October, 2014.

Party sources insisted at the time that Davidson was not the subject of the investigation.

She has since said she had not attended any postal vote opening sessions adding that there were other people who "are mandated" to do that.

On televised coverage of the referendum results, 45 minutes after the polls closed, Davidson said that the No camp had been "incredibly encouraged" by the results of a "sample opening" of the postal ballot that she said had taken place around the country over the few weeks prior to the poll.

Complaints over her account of the postal vote "tallies" raised concerns the information may have helped inform the No campaign's decision to issue the "vow" of more powers for Scotland and help the No campaign target areas of Scotland that were in danger of falling to Yes.

Around 800,000 people, or 19% of participants, voted by post in the referendum.

Police Scotland has now confirmed for the first time that "three individuals were spoken to as witnesses and as subjects of the investigation. Statements were only noted from the three individuals."

They confirmed that they were unable to elaborate the reasons for their conclusion at the end of the year-long probe as it could jeopardise any future legal proceedings "should further relevant information become known".

Police Scotland has declined to confirm the identities of the three.

In explaining why there were restrictions in disclosure of information about the investigation and how they reached their conclusion, Police Scotland added: "Whilst no individuals have been reported at this time, there is the potential for new information that could come to light that would trigger a related investigation, and as such,the release of any data at this time could substantially prejudice any future case."

Police Scotland added that they “did not report any suspects in relation to offences".

Complaints sent to police and the Electoral Commission surrounded agents being able to see the ballot papers and communicate how the vote was going in advance during opening sessions across Scotland in the days running up to November 18.

According to Elections Scotland guidance, referendum and postal ballot agents present at the opening of postal votes "must observe the requirement of secrecy".

Failure to observe the requirement is a criminal offence punishable by law and liable on summary conviction to imprisonment for a term not exceeding 12 months and/or to a fine of up to £5,000.

Police Scotland added: "The requirement for secrecy in postal votes is set out under section 66 of the Representation of the People Act 1983. Whilst defining the offence this section also identifies who may commit the offence and what requires to be proved to establish that the crime is complete. Without these thresholds having been achieved a person who is investigated in relation to this offence cannot be said to have committed a crime."

Political agents and campaigners are allowed to oversee the postal vote opening sessions undertaken up to two weeks before ballots close, where checks are made to verify the signatures and dates of birth on postal voting statements against computerised records.

However, Elections Scotland instructions on postal votes in advance of the referendum states that it is "an offence for anyone attending the opening of postal votes to attempt to ascertain how any vote has been cast or to communicate any such information obtained".

Police Scotland previously also confirmed they had received a complaint relating to separate comments made on television by John McTernan, a former Labour Party adviser in Scotland who supported Better Together.

In an interview four days before the polls closed, McTernan, a Labour Party adviser said "postal votes are running very strongly towards No". He later said his conclusions were just "a prediction".

Within minutes of polls closing then Labour spin doctor Susan Dalgety boasted online that postal results were "very positive for No".

Humza Yousaf, the then Scottish minister for external affairs, also made reference to indications from "postal ballot sampling" in another interview but the SNP said he was referring to a situation after the ballot had closed.

In video footage of Davidson talking about the ballot viewing sent to police, she says: "Postal votes are going to be enormously important in this campaign - about 18 per cent of the vote is going to come out of postal ballots and we have had people at every sample opening, around the country, over the last few weeks, while that's been coming in. And we've been incredibly encouraged by the results."

Later, referring to postal ballots, she said: "Different local authorities have had openings around the country", adding, "there's people in the room that have been sampling those ballot boxes that have been opened and have been taking tallies and the reports have been very positive for us".

Ms Davidson had previously hit out at criticism of her position in the police investigation in the wake of potentially defamatory assertions about her role on Twitter.

She called on one writer to remove one assertion saying: "There is no suggestion that I have contravened the referendum act in any way. You're a hack, check before print."

She added: "False to say police have 'launched a probe into me' as the tweet says. Careful."

She also told independent MSP John Finnie to "be careful with your retweets".

And in what was believed to be a reference to a previous Herald article added in another tweet: "As the piece makes clear there is no suggestion I have done anything wrong."

It came as Ms Davidson received a barrage of social media criticism over her position in the probe.