A private cyber security firm has been involved in a year-long probe into the anonymous Twitter accounts of officers and staff within the Police Service of Northern Ireland allegedly involved in online racist abuse and trolling.

The firm was hired by the PSNI last year to help trace the identity of the individuals behind a number of Twitter accounts, which also appeared to publish information about ongoing police operations.

On Monday, the PSNI confirmed that Twitter activity by persons purporting to be police officers was under investigation.

This morning it emerged that an internal police probe was first launched 12 months ago, shortly after the PSNI's chief constable received online criticism for comments he made on social media to an officer.

George Hamilton apologised after he told an officer on Twitter in August last year to stop "wallowing in self pity" and "dry your eyes" after he raised concern about the increasing pressures of the job.

Following the online exchange, a number of anonymous Twitter users, suspected of being police officers and staff, criticised the chief constable and posted complaints about the job and PSNI management.

A fake recruitment video was also made and posted on YouTube by one of the Twitter users under investigation.

The video shows a number of PSNI officers dancing beside a police helicopter and at other sites across Northern Ireland.

A voiceover says: "Are you considering a career where every day you drive home you fear you will be shot? Do you want the rush of wondering if the IRA has planted a bomb under your car? Do you want to be told to dry your eyes when asking for mental health support?

"Then join the Police Service of Northern Ireland today. Recruitment is now open to a host of new officers to be belittled by senior management teams, spat on by the public, harassed by the police ombudsman and have their spirits crushed."

A security source told the Press Association it was around this time that the private security firm was asked to help trace the suspected police officer owners of a number of Twitter accounts.

The source said the account that began to cause the greatest concern to management was being run under the Twitter handle @DonYeeoo because of alleged racist and sectarian comments, trolling of other social media users and sharing of classified police information.

When it first came to the attention of the authorities the @DonYeeoo account was called Fenian McTaigerhun.

The description of the account holder said "Crime Solver & lion tamer."

Some of the tweets under investigation, which have since been deleted, included: "Saudis in general just are disgusting people. Intelligence and money means nothing. Horrible dirty culture."

Another tweet said: "White people at Black Lives Matters protests are the worst of the worst .... and should throw themselves into the sea."

The Twitter user also criticised serving police officers posting: "Every now and again I run into cops who really shouldn't be cops. At one stage they were probably alright; but they checked out years ago."

A source close to the investigation said it was difficult to trace the identity of @DonYeeoo because the Twitter user knew how to cover their online footprint.

However, the source said photographs of locations within PSNI buildings that the Twitter user had posted, as well as data assistance from Twitter, have been used to trace the person suspected of running the account.

"The initial investigations into the accounts brought the attention on the @DonYeeoo account and that account became the number one priority. Some of his content was causing major problems.

"He was smart using VPN [virtual private network] etc and it took a while for Twitter to provide information necessary. He started getting sloppy. Location pictures led to his downfall. CCTV was played back at the locations," the source said.

A number of the anonymous accounts which formed part of the internal probe were suspended last week.

It is understood investigations are still ongoing.

PSNI Assistant Chief Constable Mark Hamilton recently said police are "examining material posted on personal social media accounts by a number of individuals and are undertaking investigations to establish if the individuals are all serving officers."

On Monday, the PSNI said the organisation "will not accept any racist, sectarian, sexist or homophobic behaviour from any of our staff."