Detectives have been granted additional time to question a man widely named as the British army's notorious IRA agent Stakeknife.
A team of independent detectives investigating claims of murder, kidnap and torture dating back to the 1970s, detained a 72-year-old at an undisclosed location on Tuesday, a statement from Operation Kenova said.
Officers have subsequently been allowed a further 36 hours to question the man.
In 2003, Stakeknife was widely named as west Belfast man Freddie Scappaticci but he has always strongly denied the allegation.
Bedfordshire Police chief constable Jon Boutcher is investigating the high-ranking army mole who reputedly led the republican organisation's "nutting squad", an internal security unit which brutally interrogated and murdered suspected spies during the Northern Ireland conflict.
Detectives are looking into more than 50 murders.
As well as multiple murders, the investigation team is examining evidence of other alleged offences committed by Stakeknife during the Troubles, including attempted murders and unlawful imprisonments.
The Guardian on Wednesday reported that one of the offences over which Mr Scappaticci was being held was the 1986 IRA murder of the Ulster Volunteer Force killer and gunrunner John Bingham. The IRA shot dead Bingham inside his home in north Belfast in retaliation for an increase in UVF sectarian attacks against Catholics in the area.
A number of Police Ombudsman for Northern Ireland investigators are understood to be present following the arrest.
The independent ombudsman investigates current and former police action in Northern Ireland. – PA