Why was Nicola Sturgeon asked if she is 'to be interviewed by police'?

“One factual point,” he said: “Have you been or do you expect to be interviewed by the police?”

In response, the First Minister said she was “not going to discuss an ongoing police investigation”.

“I’m not going to do that on any issue and I’m not going to do it now.”

Campbell later said on Twitter that the answer to his question was “no”.

But what was it in reference to?

Why would the First Minister be questioned by police?

In July 2021, the police launched a formal investigation, named Operation Branchform, into potential criminality involving SNP fundraising.

The force, which had been assessing a fraud allegation from a member of the public – understood to be controversial activist Sean Clerkin – since March of that year, said it had escalated its work after consulting with prosecutors.

The move followed the SNP raising more than £660,000 specifically to fight an indyref2 campaign, but spending some of the money on other things in the absence of another independence vote.

The party pledged to spend an “equivalent" sum on a second referendum.

The SNP’s annual accounts for 2019 showed the party had £97,000 in the bank and £272,000 in reserves, raising questions of where the money raised for indyref2 campaigning had gone.

SNP treasurer Colin Beattie issued a statement saying the Referendum Appeal Fund had a balance of £593,501 that could be deployed “instantaneously”. 

SNP accounts for 2021 stated that, up to December 31 of that year, a total of £740,822 had been raised “through the independence-related appeals”.

The accounts further stated that up until the same date, a total of £253,335 had been set against this income, while the remaining balance of £487,487 had been “‘earmarked’ for independence-related campaigning”.

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A spokesperson said that the funds had been spent on “extensive” research into public attitudes to independence, as well as producing campaign materials such as the effort to deliver one million pro-independence papers to households across the country.

The row over what has become known as “the missing £600,000” has prompted some resignations within the party, including Douglas Chapman quitting after a brief spell as national treasurer and Joanna Cherry standing down from the party’s ruling national executive committee.

SNP president Michael Russell (below) has previously said that the £600,000 is "not missing" and that the money is still available for this purpose.

The former constitutional affairs secretary was asked at a National Roadshow event in 2021 about the matter.

Russell said: "Colin Beattie ... has given a very comprehensive and clear statement of where that money is, that money is available for campaigning for independence, it is accounted for minutely and it will be spent on independence.

"The procedures used for it were exactly the same procedures we have used for all previous similar funds and there was nothing suspicious or devious about it whatsoever. One of the things we do suffer from is a malice from some people which goes on repeating things even when they are not true.”

A Police Scotland spokesperson said: “A report which outlines enquiries already undertaken and seeks further instruction has been submitted to the Crown Office and Procurator Fiscal Service.

“We are working closely with COPFS as the investigation continues.”