Stakeknife: Fred Scappaticci interviews monitored
Investigators from the Police Ombudsman are monitoring police interviews with the man alleged to have been the Army's highest-ranking IRA agent.
It is understood they may question him directly.
Fred Scappaticci was arrested on Tuesday morning and is being held at a secret location. He is being questioned about allegations that he was involved in dozens of murders.
Police were granted an additional 36 hours to question him.
West Belfast man Mr Scappaticci, 72, has been named as the agent codenamed Stakeknife.
Who is Stakeknife?
Fred Scappaticci is alleged to have been the most high-ranking British agent within the Provisional IRA, who was given the codename Stakeknife.
He was the grandson of an Italian immigrant who came to Northern Ireland in search of work.
He has admitted, in the past, to being a republican but denies claims that he was an IRA informer.
He is believed to have led the IRA's internal security unit, known as "the nutting squad", which was responsible for identifying and interrogating suspected informers.
Mr Scappaticci left Northern Ireland when identified by the media as Stakeknife in 2003.
The investigation into the alleged activities of the man believed to have been the Army's highest-ranking agent in the IRA is known as Operation Kenova and is being led by Chief Constable Jon Boutcher, from Bedfordshire Police.
It is examining the activities of current and former police officers, members of the Army and MI5 and former members of the IRA.
The operation involves a team of 48 detectives from police forces in Britain because some of the allegations being investigated involve current and former police officers in Northern Ireland.
The inquiry is being funded by the PSNI, but Chief Constable George Hamilton has said it he does not have any direct control or input.
A statement by Operation Kenova said it had so far spoken to more than 40 families and generated 1,500 lines of enquiry.