A man widely named as the Army's notorious IRA agent Stakeknife has been released on bail.
Detectives have been questioning Freddie Scappaticci from West Belfast about offences including murder and abduction since Tuesday.
Mr Scappaticci has always strongly denied claims that he is Stakeknife, a high-ranking military mole who reputedly led the republican organisation's "nutting squad", an internal security unit which interrogated and murdered suspected spies during the Northern Ireland conflict.
A team of independent detectives probing claims of murder, kidnap and torture detained the 72-year-old at an undisclosed location on Tuesday, a statement from Operation Kenova said.
Bedfordshire Police Chief Constable Jon Boutcher is leading the investigation.
He said: "This arrest was a significant step in what continues to be an incredibly complex and wide-ranging investigation.
"My team is continuing to speak with witnesses and victims' families and we are exploring a huge number of lines of inquiry."
He said the suspect was released on bail to return to police custody at a date in the near future.
The chief constable added: "I cannot stress enough how important it has been for people to come forward to speak to the investigation team. It is very clear to me that there are many who see now as the right time to speak and help us get to the truth for the victims' families.
"They deserve to know what happened to their loved ones.
"I ask that anyone, from whatever organisation or community, who might be able to contribute to that truth to come forward to speak either to me directly or the team.
"Anyone who contacts Operation Kenova will be treated with great care and respect."
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.
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.