Police have been given a further 36 hours to question the man alleged to have been the Army's highest ranking IRA agent.
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.
West Belfast man Mr Scappaticci, 72, has been named as the agent codenamed Stakeknife.
The investigation team said the arrest was "in connection with the investigation into allegations of murder, kidnap and torture".
In a further statement on Wednesday, they said: "Officers from Operation Kenova have been granted additional time to question a 72-year-old man who was arrested yesterday (Tuesday) in connection with the investigation.
"The man was arrested on suspicion of a number of offences. Detectives were today granted a further 36 hours to question him.
"He remains in custody at an undisclosed location. No further details of the place of arrest or where he is being held will be released due to security reasons."
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.
Operation Kenova 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.
A statement by Operation Kenova said it had so far spoken to more than 40 families and generated 1,500 lines of enquiry.