The chairman of the Garda Síochána Ombudsman Commission (GSOC) says he suspects the organisation's office was under some sort of surveillance.

Simon O’Brien also told the Oireachtas Public Service and Oversight Commission officials within the Ombudsman’s office believed some threats came from the An Garda Síochána themselves.

However, he stressed there was no evidence of surveillance of GSOC by the Garda.

While he regretted not informing Minister for Justice Alan Shatter of the issue, Mr O'Brien said he made a "strategic decision" on his own not to do so.

“The level of public disquiet in allegations that Gardaí might be involved in any kind of that activity was immense”, Mr O’Brien said.

Disclosing the information also “had immense potential for further damage to what was already a strained relationship” between GSOC and the Garda Síochána.

He added that GSOCs’ director of investigation believed last October that threats came from members of the force.

The decision to carry out a security sweep of the GSOC’s Dublin office “came from nothing more than the thoughts” the risks of a security breach were increasing and the GSOC needed to be aware of the risks.

However, the threats discovered were on the “higher level of suspicion”. “There is a very, very small chance that this could be some innocent telephone call coming in.”

The context of the sweeop was a higher public profile for the GSOC because of differences with An Garda Siochana, as well as more information about GSOC entering the public domain.

A group of only four or five people were aware the sweep initially, but this grew to a “cohort” of around ten.

“We kept the security sweep to a very tonight group of individuals, I think no more than four or five.”

Mr O’Brien said he “strongly suspects” a section of a confidential report on the sweep was given to a journalist, and he had the report in his possession since before Christmas. He said the initial story in the Sunday Times newspaper seemingly had a strong basis but claimed there are inaccuracies in the confidential GSOC report about the security sweep.

“I have looked at that document and that does not fit a recollection I have,” Mr O’Brien said.

“It is highly likely that information has come from documents within GSOC,” he added, with “less than seven” people having access to the documents.

“It could be that the person who leaked this report is responsible for other issues.” However, he said the confidential report may make reference to reasons “for the sweep that do not accord with my recollections as the reason for the sweep”.

He also regretted Mr Shatter and Garda Commissioner Martin Callinan had been "blindsided" by the controversy but said he would be furious if he were in Mr Callinan's position.

Mr O’Brien met with Mr Callinan earlier this week to discuss how GSOC and the Gardai could move on.