Detectives have been unable to catch the culprit and the probe is now at an end. But an insider at the Better Together campaign said the police statement disproved Yes Scotland's original claim that its email system had been illegally accessed.
Last year, Yes Scotland campaign chief executive Blair Jenkins called in the police in the belief that a senior employee's email had been hacked.
Campaign staff suspected that an email showing that academic Elliot Bulmer had been paid, controversially, to write an article for The Herald newspaper, may have been obtained illegally.
However, Yes Scotland staff were accused of making the complaint in order to create a diversion to take attention away from the Bulmer payment.
The pro-independence campaign had initially referred to "Yes Scotland email", with Jenkins adding that there was "illegal access to our communications system, to our email system, here at Yes Scotland".
The organisation then clarified that their concerns centred on the personal email account of a Yes staffer. This newspaper later revealed evidence of how an anonymous individual had hacked the personal account.
On the day it was revealed the police had been called in, an individual sent an email to Jenkins, writing: "If you want to rein in the officials and in return be granted a full disclosure of my 'intrusion' then I shall be more than happy to comply. In addition, you can be assured that there will be no publication of documents. Let me know if there's a compromise to be reached."
In another email, the individual wrote: "My offer to disclose all remains, thereby ensuring you have full comfort and control of all the facts. My find was purely by accident, however, I will have absolutely no hesitation in releasing this in full.
"There is sufficient public interest for disclosure and that in itself would work to my advantage."
Hours later, members of the Yes advisory board received an email from the personal account at the centre of the Bulmer row, but the staff member whose account it was had not written the email.
The email stated: "As previously discussed, this is not a hack. This is yet another exposé of the poor security and user practices in place by individuals associated with Yes Scotland.
"Please do ensure that the advisory board is aware of all the communications to date, where I have sought to reassure."
The mystery individual added: "Do ensure all are aware that I've approached Blair many times within the past 24 hours, but no response …"
Board members include Deputy First Minister Nicola Sturgeon, former Labour MP Dennis Canavan and Scottish Green MSP Patrick Harvie.
As part of the inquiry, detectives probed claims that the hacker was based in either Ireland or the United States.
It is also understood officers felt frustrated at the lack of co-operation from overseas telecommunications firms.
The probe failed to identify and apprehend the individual.
A spokesperson for Police Scotland said: "Officers from Police Scotland conducted a full and thorough investigation, in both Scotland and overseas [Europe and America] in connection with unauthorised access of an email account and those enquiries are now complete.
"The person who accessed the email account has not been identified. However, should additional information come to light, that will be further investigated."
A spokesman for Yes Scotland said: "We appreciate the time, effort and professionalism the officers of Police Scotland invested in what has been a very difficult and protracted investigation - more than eight months, in fact. Regrettably, the hacker remains at large.
"It is to be regretted that there was a reluctance by digital service companies in two countries outwith the UK to co-operate fully with requests from the Scottish detectives."
A Better Together campaign insider said: "This has been a sorry episode. In order to try and deflect attention from the fact that the Yes camp were paying people to write nice things about them, Blair Jenkins concocted a story about their email system being hacked and then sent his spin doctors out to whisper about who was behind it all. This statement from Police Scotland exposes the truth. The Yes Scotland email system was not hacked. There were no 'force or forces unknown.' There was no 'assault on democracy'."