A former investigator with the Police Ombudsman's office has been arrested.
The arrest forms part of a PSNI (Police Service of Northern Ireland) investigation into the alleged theft of sensitive security documents.
It is understood the documents contained information the police and security service MI5 believe could put the lives of individuals at risk.
The BBC understands they were released to lawyers without authorisation from the ombudsman, or the PSNI.
The police have suspended the release of any further sensitive material to the ombudsman's office until a review of security protocols is carried out.
The Police Federation for Northern Ireland said it was an "astounding and very worrying state of affairs" and has called for a full inquiry into the operation of the ombudsman.
A 69-year-old man was arrested in Dartford, Kent, on Sunday after detectives from the PSNI's Serious Crime Branch conducted a joint search with local police.
It is understood he retired from the ombudsman's office a number of years ago and that the documents do not refer to any case currently being investigated by it.
This is hugely embarrassing for the Police Ombudsman.
It's not known if the documents released to lawyers contain the names of informers or suspects under investigation, or both, but it's understood that the documents are regarded as historic and not part of any current investigation.
The fact that the PSNI statement was issued by the Assistant Chief Constable who heads its Crime Operations Department, and not the officer leading the actual investigation, is a clear indication that the issue is being taken very seriously.
There have been huge disagreements between the Ombudsman and the PSNI and MI5 over access to sensitive security material.
Some within the police and security services have strongly resisted requests for access to highly-sensitive material and expressed concern that it could be leaked. The Ombudsman's office has dismissed those concerns and pointed out that investigators with access to such information have high-level security clearance.
However, this incident will strengthen the arguments of those who want to limit the amount of material shared with the Ombudsman. That would seriously undermine the Ombudsman's ability to investigate cases where there are serious allegations of wrongdoing by current or former police officers.
In a statement to the BBC, a spokesman for the ombudsman confirmed he was alerted by police last week about the theft of sensitive material originating from the office.
"The material came to light during legal proceedings not connected to the office," he said.
"We immediately asked the police to begin a criminal investigation into how and when this happened. We also informed the Information Commissioner."
Assistant Chief Constable Steve Martin, head of the PSNI's Crime Operations Branch, confirmed a man had been arrested as part of the investigation.
'Element of fairness'
"The PSNI can confirm that it has become aware of a suspected theft of sensitive documents from within the Office of the Police Ombudsman of Northern Ireland," he said in a statement to the BBC.
"We have now commenced a criminal investigation and are also carrying out an assessment of any impact which may be caused by the unauthorised release of sensitive material.".
The statement added that the police and Police Ombudsman's office have agreed to "a temporary period of review during which sensitive information will not be shared."
In its statement, the office of the Police Ombudsman said it was anticipated that this suspension of the sharing of sensitive material "will be temporary."
The Police Federation for Northern Ireland said there had to be an element of fairness for police officers under investigation.
"They need the same protections that a normal member of society would have and that doesn't exist at the moment with the ombudsman's office," Mark Lindsay, the representative body's chairman, told the BBC's Good Morning Ulster programme.