A police probe into allegations of a £600,000 SNP donations fraud has been codenamed Operation Branchform by detectives.
The investigation was launched last year after it was claimed money had been illegally diverted from a “ring-fenced” fund to fight an independence referendum.
The row over the “missing” cash sparked the resignation of several senior nationalists from the party’s ruling body.
It was claimed SNP chief executive Peter Murrell, who is married to Nicola Sturgeon, had refused them permission to see the party’s accounts.
The party has not said if either have been interviewed by police, but criminal complaints from at least 19 people have now been received.
Sean Clerkin, the campaigner who made the first formal complaint, said: “It seems clear to me that this was money that people were told would be used to fight a referendum campaign and it was spent on other things.
“The police need to step up this investigation. It seems like a relatively straightforward case to decide whether any criminal charges are required.
“What they have told me is that the investigation is very much active and that it has been given the code name Branchform.
“I have no idea whether that is a reference to the form that independence activists filled in to donate money, but they were clear that is what it has been called.”
Senior officers at Police Scotland are reported to believe the complaints have some substance and that criminality may have occurred.
However Crown Office officials are rumoured to have argued that all spending by the party ultimately supports the aim of independence, and that a crime is unlikely to have occurred.
Financial documents were handed to police by the SNP shortly before Christmas after officers executed a warrant authorised by the Crown Office.
The Sunday Mail first revealed the police probe in April last year. It was initially denied a formal investigation was taking place, but this was later confirmed.
It came after Douglas Chapman, MP for Dunfermline and West Fife, quit as national treasurer after complaining he had “not received the support of financial information”.
Edinburgh South West MP Joanna Cherry also stood down from the party’s ruling national executive committee over “transparency and scrutiny concerns”.
Three other members resigned from the party’s finance and audit committee after claiming Murrell was refusing to open the books.
The dispute centres on funds raised in 2017 and 2019.
Fundraising documentation said the cash would be “ring-fenced” and only spent on a referendum campaign.
However the SNP’s accounts showed less than £100,000 in the bank at the end of 2019, despite the fact a referendum had not been held.
A source said: “Not much is being said about it but the investigation is very much active and interviews have been carried out.
"It is difficult to believe that the police will not have interviewed senior figures in the SNP by now.
“Whether it goes anywhere remains to be seen but the First Minister and her husband must be very worried about things escalating.”
The Crown Office said it would not comment on an ongoing investigation.
The SNP was approached for comment.
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