A man widely named as the British army's notorious IRA agent Stakeknife has been arrested by detectives, sources said.
Freddie Scappaticci has always strongly denied the allegation.
A team of independent detectives probing claims of murder, kidnap and torture detained a 72-year-old at an undisclosed location yesterday, a statement from Operation Kenova said.
Bedfordshire Police Chief Constable Jon Boutcher is investigating the high-ranking British army mole who reputedly led the republican organisation's "nutting squad", an internal security unit which brutally interrogated and murdered suspected spies.
Dozens of detectives are probing more than 50 murders. In 2003, Stakeknife was widely named as the west Belfast man Scappaticci, but he has always strongly denied the allegation.
A statement from the police investigation said: "Officers from Operation Kenova have arrested a 72-year-old man in connection with the investigation into allegations of murder, kidnap and torture."
The team of detectives began investigating after the former director of public prosecutions in Northern Ireland, Barra McGrory QC, referred multiple allegations to the police.
The PSNI decided to call in external help from officers outside Northern Ireland.
The investigation update added: "Operation Kenova is a complex and wide-ranging investigation, which was launched in June 2016 to investigate allegations of murder, kidnap and torture dating back to the 1970s.
"So far, the team has engaged with more than 40 families and processed more than 500,000 pages of information, generating 1,500 lines of inquiry."