STAFF at SNP headquarters are set to be re-interviewed by police investigating the finances of the party, it has been reported.

Workers have been sent letters which ask then to speak to officers, with the move being directed by the Crown Office, according to The Times.

It includes staff who were not in place when the inquiry began. 

Operation Branchform, the probe into the SNP finances, has now run for more than two and a half years.

READ MORE: Operation Branchform: SNP investigation costs 'now over £1 million'

It has seen the high-profile arrests of former first minister Nicola Sturgeon, her husband Peter Murrell, former SNP chief executive and Colin Beattie, the party’s former treasurer. All were released without charge pending further investigation.

Sources told The Times that Murrell has been keeping an extremely low profile. 

One senior figure said: “Nobody talks about him, nobody has seen him, nobody knows where he is."

READ MORE: Lawyers call for explanation of why SNP finance probe taking so long

Operation Branchform is looking into what happened to around £600,000 raised by the party for independence campaigning.

Last July then chief constable of Police Scotland Iain Livingstone suggested that the investigation had "moved beyond" the initial complaint and said that the time taken over it was "absolutely necessary".

At the end of last year it was reported the probe had so far cost more than £1 million and two former sheriffs called for clarity on its progress.

In January First Minister Humza Yousaf  addressed the issue of the police investigation into SNP finances, saying it “clearly” affected the public perception of the party.

He said the inquiry has been “one of the most difficult times” for the SNP and that he must work hard to rebuild trust.

A spokesman for the Crown Office and Procurator Fiscal Service said: “Senior professional prosecutors from COPFS and an advocate depute are working with police on this ongoing investigation.

“It is standard practice that any case regarding politicians is dealt with by prosecutors without the involvement of the law officers. All Scotland’s prosecutors act independently of political interference. As is routine, to protect the integrity of ongoing investigations, we do not comment in detail on their conduct.”

An SNP spokesman said: “As we’ve said previously, the SNP has been co-operating with the inquiry and will continue to do so; however, it is not appropriate to comment further on a live investigation.”