A man widely named as the 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 on Tuesday, a statement from Operation Kenova said.
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.
In 2003 Stakeknife was widely named as west Belfast man Scappaticci but he has always strongly denied the allegation.
A statement from the police investigation said: "Officers from Operation Kenova have today (Tuesday) arrested a 72-year-old man in connection with the investigation into allegations of murder, kidnap and torture.
"The man has been arrested on suspicion of a number of offences which are being investigated by Operation Kenova."
The independent team of detectives began investigating after the former director of public prosecutions in Northern Ireland, Barra McGrory QC, referred multiple allegations to the Police Service of Northern Ireland (PSNI).
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.
"Anyone with any information relating to Operation Kenova can contact the team by calling 01234 858298 or by email email@example.com "
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.