The man widely named as the Army's IRA agent Stakeknife has been released on bail.
West Belfast man Freddie Scappaticci was arrested on Tuesday in connection with a number of offences including multiple murders, attempted murders and abduction.
A team of independent detectives is investigating the alleged offences which date back to the 1970s.
Scappaticci - who is 72-years-old - has always strongly denied claims that he is Stakeknife.
He was released on bail on Friday to return to police custody at a later date.