The police investigation into Peter Murrell’s fraud took years to complete partly because the SNP’s accounts system was so ‘difficult to unpick’, a senior officer has said.

Assistant Chief Constable Stuart Houston said Operation Branchform, which cost more than £2million, was ‘complex’ and the party’s ex-chief executive had been skilled at covering up his fraud.

He said Police Scotland’s specialist crime division (SCD) had been key to its success.

At a public board meeting of the Scottish Police Authority in Glasgow, Mr Houston defended the cost of the investigation, which began in 2021.

He said: ‘I would go back to the complexity of this. It spanned a 12-year period [covering] someone who has been continually covering their tracks in an accounting system that was difficult to unpick.

‘You’ve probably all seen the number of items [paid for with embezzled cash] in the indictment. There had to be a thorough investigation to evidence every part of that and get us to the position where we got a guilty plea.

‘There’s a lot of work that’s gone into that – and a long-term commitment by the team.’

He said it involved ‘going through financial documents that required an accurate understanding of such complexity’, adding that a ‘huge effort was put in jointly with the Crown Office and Procurator Fiscal Service [COPFS] to ensure the best possible evidence was obtained’.

Mr Houston, who led the probe, added: ‘We have a real benefit in Police Scotland with the SCD, where we have investigators of a high standard where we can flex across different investigations.

‘With this particular inquiry, you had a need for operational security, integrity and protection of what was ongoing – that was the challenge of this case and that did require us to have a core team.

‘Although a police inquiry is complex, it should be pointed out that working alongside the COPFS was important to ensure we were matching their commitment, what they were putting in, their preparation and assistance and guidance throughout the inquiry.’

Chief Constable Jo Farrell told the meeting that the investigators ‘worked under intense scrutiny’ in a case with ‘significant public interest’.

Murrell, 61, admitted embezzling more than £400,000 from the SNP over 12 years, during which time he was the party’s chief executive.

Ms Farrell thanked Mr Houston, as well as other senior officers in the investigation and Police Scotland’s financial crime unit. ‘Their efforts led to this week’s guilty plea by Peter Murrell, who abused his position to divert SNP funds to his own account to fund luxuries,’ she added.

Graeme Biggar, director-general of the National Crime Agency – nicknamed ‘Britain’s FBI’ – which supported Operation Branchform, said it had been a vindication of the single force, showing it was not politically influenced.

At the meeting, he said: ‘When Police Scotland was being considered back in the early 2010s, one of the concerns was that if you had a single national police force, it wouldn’t be able to get too close to the government – and it wouldn’t be able to hold the government to account, to investigate the government when they needed to.

‘One thing that Branchform has shown – and there’s no doubt of the complexity that’s been involved in this – is that that has happened, and where a crime has been committed, the police have been able to hold that individual to account.’