Detectives have been granted additional time to question a man widely named as the Army's notorious IRA agent Stakeknife.
A team of independent detectives probing claims of murder, kidnap and torture 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.
Dozens of detectives are probing more than 50 murders.
Almost 50 detectives have been working on the Stakeknife investigation.
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.
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.